Xbox and ethernet

I bought a used Xbox for my stepson and it works fine but won't connect to the network through the ethernet cable. His older one did fine and this one will at his dads through a modem. I have a AT&T uverse Gateway router and high speed. The ethernet connection light does not come on on the gateway and the Xbox says there is no network connection. I manually entered the IP and subnet mask with no luck, called Xbox and they were useless and just wanted me to send it to them and fix it for 125.00. Anybody have any idea, I feel real bad that he can't get on live. Tappon1