Were you able to successfully authorize the Plex app for the TV? Can you access your server on the computer fine?
Some wi-fi repeaters will create separate networks with their own DHCP addressing separate of the router they are connected to. You would want to check the settings on your repeater and make sure it is set to “bridge” mode. Also check the IPs of the Fire TV and the server to confirm they are on the same network addressing.
If your server is on windows, make sure the network connection is set to private and not public. Since you moved and are now on a new network, it may have switched back to public. Public will block local devices from connecting directly.
Also, confirm the WiFi network doesn’t have any device isolation settings or is a guest network.