The most likely issue is that you are using the wrong cable to connect the modem and the router. You should a crossover cable, not a patch cable. Do I have to explain, or is this enough?
After installing a new DSL modem, I couldn't get my router to work properly despite checking all the connections, settings, etc. I figured out that if I connect the modem into one of the 4 LAN ports, everything works just fine. Everything I've read or seen says to plug the modem into the WAN/Internet port so is something wrong with my router?