WNDR3700v2 One port not working

One of the LAN port? Only thing to check is with flash light look at the cable socket to see if any springy pins are out of it's slot or broken.
Other than that not much else to check. Once I had that problem when some one knock over cable damaging the socket on my laptop.

Had some difficulty to replace it but I managed to do it.  If you are running out of port, just get a cheap dumb switch. They don't cost much.