Need a way to connect multiple devies to a legacy router with lower latency

There are 2 possible problems here - the issue could be caused by 1 or both.

Specifically, if you are having problems using more then 8 devices the issue could be with the firmware in the router. Very often cheap devices are not good at doing connection tracking. If this is the problem then the solution is to either replace the device with a better router or turn the router into a bridge and put a decent router in place. From a POV of connection tracking issues, if you are getting a second router any ethernet router that can run dd-wrt will do - dd-wrt does not have issues with lots of clients and is typically more robust then the software routers come with.

The Second issue - and this is where my money is - is the WIFI. WIFI is convenient - and thats about all it has going for it. It is not reliable, and it is not (compared to wired connections) stable. It is also half-duplex. Also, in anything other then a farraday cage it is impossible to get anywhere near the quoted speed of WIFI. It is unclear from your question if you get 80 mbps from your ISP or reported by your WIFI. If its reported by your WIFI, that 80mbps is shared between all devices on your network (and 2 WIFI devices mean less then 40 mbps each or less)

As a rule, anything you can cable with ethernet you should. This will make those devices more reliable and faster, and will reduce the amount of use of the WIFI spectrum - providing more spectrum for those devices that need it. (Servers don't belong on WIFI).

MIMO can help, but is not a panacea.

The assertions which you made which I believe are wrong include -

• The routers compatible with DSL connection are pretty old and don't support new techniques for faster connection to multiple devices like MIMO. There are plenty of these - to throw out the first 2 links I found - https://www.tp-link.com/au/home-networking/dsl-modem-router/archer-vr2800/ and https://www.asus.com/nz/Networking-IoT-Servers/Modem-Routers/All-series/DSLAC68U/ (If you are limiting yourself to ADSL routers, thats your problem - look for VDSL modems and just double check they work with ADSL - almost all will)

• DSL is not a very old technology. ADSL1 - ie the first varient is. VDSL2-VPlus was only approved in 2015, and of-course was rolled out later. To be contentious, in Australia VDSL connections are actually called fibre connections, and is the currently used method to deliver fibre to many locations - even in dense areas.

• You don't need a static IP from your ISP to monitor your servers (see below)

• Latency issues are not caused by lack of MIMO (although MIMO can mitigate the issues a bit), it is caused by using unregulated spectrum on a shared half-duplex media.

To answer your question -

An AP is a device which converts between ethernet and WIFI. When used as an AP, the AP/Router becomes invisible in terms of traceroutes and IPV4/IPV6 connectivity, so it does not introduce any NAT issues.

You can convert most routers into AP's (access points), and plug these into a router. To do this you connect a LAN port on the main router to a LAN port on the the Router/AP, and disable DHCPD.

Using ethernet [what you call LAN cables] (and a switch) to connect your router to the server is a good idea and a best practice for servers. If you do this, try to only use 1 LAN port on the router to connect it to the switch and feed it from there. A dirty secret of SOHO routers is they use software to create a virtual switch on the 4 ethernet ports, and this can impact performance.

Suggestion

I think you have asked the wrong question. If you have both DSL and 2 ethernet connections, I put to you it would be better to re-evaluate your network and rebuild it from the ground up to support your goals.

I note you say you have DSL only for the static IP address for monitoring your servers. You also complain about, in effect, a crap DSL router. I would solve both of these issues by ditching the DSL connection and router, and getting a decent ethernet router which will match your needs - and, if done smartly, will be able to balance your 2 fibre connections.

There are (at least) 2 ways I can think of to bypass the dynamic IP issue on your fibre connections -

• I would put an OpenVPN client on a decent ethernet router running DD-WRT and purchase a VPN service that offers a static IP address. (There are lots of them - google Static IP VPN - these services can be had for less then $5 per month from reliable VPN providers)
• You could use dynamic DNS. If you are not constantly rebooting your DSL connection, this may be good enough.