Linksys WRT54GS router is not recognizing PC

I have a Linksys WRT54GS router that is giving my some trouble. I recently got a new PC and moved my old one into my bedroom along with my Xbox 360. So I didn't have to switch my Ethernet cable between my xbox and pc I bought another cable. Much to my dismay this new cable would not work. At first I thought maybe the cable was bad, but when I connect my pc directly into my modem it works fine. This had my come to the conclusion that it was my router. I have two pcs hooked into it that work fine and a laptop that connects to it wirelessly, but if I try to hook my xbox or pc to it using the new cable they just act as if there isn't a cable connected. When I look at my router the light for the ethernet port that the new cable is in will flash on and off. I've hard reset my router and updated the firmware. I've also spent multiple hours scouring the internet for solutions at no avail. If anyone can help me I won't be able to thank you enough.