Neighbor stealing Wi-Fi

My Internet connection is slow and I am (almost) sure that the neighbor is stealing it. Whenever they are home, my Internet connection is slow. I logged in to my modem, but I am unable to find any strange devices in Device Manager. Is it possible that they are hiding on my network? I also notice that when I do a modem reset the Internet speed increases, but after a while slows down again.

Thanks for the input everyone, I am reading and writing down stuff I didn’t know about, appreciate it. I noticed my previous post lacked information so I decided to add the following:

• OS: Windows 10
• Modem: Huawei hg 659 (provided by ISP, no additional router operational)
• ISP: T-mobile home DSL

I switched off all devices at home unplugged everything and shut down all mobile devices. The WiFi and LAN lights keep on flashing and won’t stop. I still have to try the cable method. During the day I have to work from home so I can’t do anything till the weekend. I will keep monitoring for further information and advice.

The other answers so far are about security, but there is another factor that may well be at least part of your trouble.

A wireless network at 2.4 GHz (in Sweden where I am, using b/g/n) allows 13 channels. (My router also has an "auto" setting.) On top of that, if the router has 5 GHz Wifi there is another set of channels.

I suggest you download e.g. Wifi Analyzer or any similar app into your phone, and use it to check which channels are most used - disregarding your own router [check which channel it uses in the "WLAN settings/setup"].

Now; Avoid those channels!

Look up any channel that is among the least used, and set your router to that - or possibly "Auto" if there is one such setting; assuming it wasn't set while you've had trouble.

Note: move about in your apartment / house and check in different locations. You might discover that the signal strength is weak where your computer is - related to the router (remedy: move the router and computer closer to each other; avoid having walls between).

Possible root cause:
If your router uses the same or a neighboring channel as any other nearby router - then your throughput will be lessened by "collisions", the more intense use, the more of it.

One more thing to think of:
If you have a lot of devices running on WiFi - and many in use at the same time; this itself may become a problem. "A lot" of active users might create such an amount of traffic that you get congestion i.e. WiFi "traffic jam".

Worth mentioning:
A microwave owen uses 2450 MHz to create the heat. This is inside a Faraday cage though - shouldn't leak much, normally.
Bluetooth devices on the other hand has more potential to cause trouble - as the frequencies used in that radio are the same as for WiFi.
The ISM Bands include the same frequencies, many more device appear here. (e.g. cordless phones, wireless headphones, car keyfobs, security cameras, zigbee devices, remote control devices(?), ...)
Yet another Wikipedia article covers the technicalities, which include some details that, to some extent, also laymen could understand. The Interference section might be of interest for anybody. Detail; Wifi uses "CSMA/CA" in contrast to "CSMA/CD" for wired networks.

Not knowing your setup there are more possibilities than a hack:

If you are using a leased line from a telco or have a dial-in line (even with UMTS/LTE = 4G/5G) the line/cell may be shared between users. So the more users the lower the bandwidth for the single user. Some telcos are very creative to limit usage for normal users. If one ore more users on a shared connection has a premium service (like IP-TV or some other "package") or pays a special fee with bandwidth minimum this user/s get/s e.g. 9/10 of the bandwidth). So this might be the case here too.

To rule out a hack connect directly via cat5/6/7 cable to the router and switch the wifi part off for a week or two. If the problem is solved that way - either get a secure router (No firmware hacks available, no backdoors known), enable strongest possible security (not WPA as it is compromised) and use strong network passwords changing on a regular basis.

But to have a real secure line you will need cable connections to the router (what goes over the air can be hacked). In my neighbourhood from ten scanned devices 9 have known weaknesses (old firmware, outdated security features, weak password, known telco/provider admin passwords, and so on). That's why I go over cable or direct encrypted LTE/VPN dial in ;-)

You will need to provide a lot more information prior to getting a truly useful reply. Otherwise, only general advice can be given. Also, you will need to work on this systematically, there is no single "press here, use that tool" recipe.

First, you must be sure where the problem is. You think the neighbour "steals your Wifi", alright, so let's formulate the problem as "something Wifi", and get back to "stealing" later.

First of all test with Wifi turned off and an ethernet cable attached to the router.

Problem gone? So the problem is really within the "something Wifi" realm.
Problem persists? Not related to Wifi, stop searching there!

Next, you need to be sure that what you see is not "perfectly normal" because you are in some way using a shared-bandwidth channel (such as every back-channel cable modem because that's just a technical limitation, the majority of 4G/5G, and a good number of cheapish DSL providers).
For example, being with a cheapish DSL provider in a third world country such as Germany can imply that it is perfectly normal if your 100MBit/s link suddenly drops to 70MBit/s for no apparent reason. That's because they acquired a few extra customers in your neighbourhood, and they didn't book enough bandwidth on the fiber backbone, so they're just reducing bandwidth silently. As long as nobody complains, they leave it there. Sometimes, reconnecting and getting a different slot in the DSLAM "magically" fixes it. Whatever. Such issues are annoying, but they can be very real, and they're unrelated to your neighbour. Same for cable modems. Neighbour starts to download porn, and your bandwidth drops. That's normal. It's just how the technology works.

Now, assuming the problem is indeed gone after using an ethernet cable, what's next. First, of course, your neighbour may indeed be stealing your internet bandwidth, but you should be sure that's acutally the case before burning down his house.

There are many ways Wifi can be slowed down (regularly, accidentially, and deliberately). First, your neighbour may have his own WiFi, and all networks on the same channel influence each other. Additionally, neighbouring channels can be, and usually are affected, too (often several of them!). Depending on channel width and clever or not-so-clever channel selection, this can range anywhere from "who cares" to "total desaster", in normal operation. And, this is entirely legitimate.

Also, devices and the type of protocol that they use on the same channel, play a role. A "typical" 2.4GHz router supports 802.11 b/g/n by default, unless told otherwise. Which is actually a really stupid choice because if you have a single "b" device nearby, you're down to 11MBit/s for all devices. Then there's microwave emitters such as actual microwave ovens, or seemingly harmless microwave emitters such as e.g. cheap LED lightbulbs, or a Playstation. Neighbour comes home, turns on light, Wifi dies. That's actually possible! Depending on what exactly it is, that is not "legitimate" in most countries, but you will have to find out first.

Now, you said that you even tried to turn on WPA, which is funny because not only is WPA very outdated (even WPA2 is on its way being phased out) but it sounds like before that you didn't have any such thing as a Wifi password at all?
Now you should know that while WPA2 is subject to e.g. KRACK, which is, in layman words, a way of eavesdropping communications, it is reasonably safe against being "stolen" otherwise. The eavesdropping is not normally a problem because the internet itself is unsafe, so anywhere it matters, you use TLS anyway. Only just, accessing your router's admin panel via Wifi (telnett, likewise), and consequently entering your password for everybody nearby to read, is not such an awesome idea for that very reason. Cable, please. Ethernet cable. Only. Ever.

Apart from that, WPA2 is reasonably safe, except, well except most routers have WPS enabled by default.
What's that? It is a functionality that makes setup more comfortable (press button method). Unluckily, it is also something that is trivial to brute-force (4-digit PIN), and for which there exist tools that let anyone break into your WPS-enabled network within a minute. Every no-shit router has an option to turn WPS on only temporarily on demand, and restrict access to already known devices otherwise. Do that. Even if your neighbour isn't stealing your Wifi, still do it. Just, because.

So you change your password and an hour later you feel like someone is stealing your Wifi again? That looks just like them exploiting WPS. Turn WPS off, change password, and see what happens. Problem gone? There's your answer.

Rule out possible causes one by one that way. If disabling WPS didn't change a thing, look how many wireless network SSIDs you see advertized in your network connections pane (or in the tray). No tools needed.
If there's like two dozen of them, it's clear why internet sucks at times. If there's just you and nobody else, it's a bit suspicious. Etc. etc.