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.

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.