It definitely sounds like a backlight issue, but check the display connector again. I had some weird video problems with my AMD main board when I upgraded, some of which sound a bit like yours. It ended up being a speck of dust or something in the connector. Once I blew that out with some canned air and reseated the connector, everything worked properly. Worth a shot at least.
This. If you’re not trying to use the wifi on the 2nd router, and it’s already wired to the 1st one, just replace it with a switch with the needed number of ports. Having a second router can definitely cause problems if the routing functions are still active.