i looked at the cross-post, do you have the spec sheets for transceivers? do you have any sort of filter / mux panel.

short between pairs probably. tdr (time domain reflectometer) cable tester should help locate fault. might be pinched somewhere or something else

there are “casts” you can get to repair buried cable. usually used in copper telephone plant, i wouldn’t trust it to certify to a gig but you could use it for a backup link or if you play with pots at some point. or “temporary repair”

i would expect your lengthy research would have included “router model default password”, “reset password router model” and “port forward on router model”