Accidentally changed an IPv4 setting on Synology NAS Cannot access it anymore

Solution 1:

Synology NASes have a networking reset procedure:

1. Locate the RESET button on your Synology NAS.
2. Use a paper clip to gently press and hold down the RESET button for about 4 seconds until you hear a beep, and then release the button immediately.

[...]

8. Disable the admin account by first signing in with your administrator account, and then going to Control Panel > User > the User tab. Double click on admin and tick the Disable this account checkbox. Click OK.
9. The admin account status should now be Disabled.

Steps 8 and 9 are necessary because this procedure will also re-enable the default admin account, which should be disabled for security under normal circumstances.

The RESET button is typically at the back, in a small hole. You can press it with a paperclip.

Solution 2:

Okay, if your Synology still has its default IPv6 setup you can try to find it via the IPv6 Neighbor Discovery Protocol. The NDP list shows only devices, which had communicated to each other. So we need to issue a multicast ping. This will trigger an answer from all members of the subnet.

We need the interface name for this. In Windows it is a number, which can be found in ipconfig. Open PowerShell and attach the output to your question:

ipconfig /all

Let's say it returns the following link local IPv6 address:

fe80::9059:69ff:fed5:8dc9%3

In this case the interface ID is 3. It can be found behind the IPv6 address, separated with a %.

Run the ping

ping ff02::1%3

Then list the neighbors

netsh interface ipv6 show neighbors

Now there should be at least one entry in the list where the IPv6 address starts with fe80:. This should be the address of the connected device - your Synology. Enter that in your internet browser with the same port (:5000/:5001).