Look at the control board. It's full of dust...

Try the mikeywith suggestion first, and then if the issue persists, do this below.

Clean the control board, then check the parts near the ethernet socket and, maybe there are some parts with rust and corrosion clean them with thinner. If it can't remove, try to resolder them. Use a multimeter to check the ethernet terminal and maybe it's shorted. If you have Hotair(BGA) you can replace the ethernet socket with the new one.

Also, try to make a crossover cable and use it as your ethernet cable.

If all of them don't work, your final solution is to replace it with a working control board.