The cat 5e that the router came with worked for a day or two and then stopped. Tried ~5 other cat 5e cables, still no. Randomly tried a 30ft cat 7 and it worked! Switched between the cat 7 and various cat 5es to make sure that it really is the cable making the difference, and confirmed that. So I bought a 5ft cat 7, and it doesn’t work.
With the cables that don’t work, both the router and modem recognize the issue. Light on the router indicates that it detects data input but doesn’t like it. Light on the modem indicates it’s sending data but something is wrong. Both manuals go in to 0 detail about what the lights mean other than that.
Modem is a Nokia with fiber input, router is a Netgear.
The root of the issue ended up being the power strip both the router and modem were plugged into. Having either or both powered via that strip caused only the 30ft cable to work. With both plugged directly into the wall outlet, any cable worked.
Weird. I’d strongly recommend getting a new surge suppressor. I’m glad you were able to figure it out though.