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.
focus on the ends, not the cables. a damaged jack may work better with a ticker cable end that the longer cable has, instead of the older cat 5 cables that may put less pressure on the connection.
sometimes the pins inside the jack can be weak and not popup after a cable is removed leaving an open connection… you can usually see this if you look.
shitty picture:
https://i.redd.it/qjfo47p5wyp51.png
if you notice the pins that arent rebounding all the way up, they will possibly make poor or no contact. sometimes you can confirm this by gently pushing down the connector to force it to get a better connection.
https://i.stack.imgur.com/kustE.jpg
shoving down with something else might help for a moment to test.
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.
thanks for the answer, might help others. line noise is so incredibly rare these days. hope you threw away that surge strip and got something else.