Ok here's one possible solution: check the default IP addresses of the router and modem.

I had this problem hooking up a router to my gf's AT&T DSL: the modem was 192.168.1.100 and the router was 192.168.1.1. That third space has to be different for each otherwise you get a conflict.

I went in to the router config and changed that third space to 192.168.(anything that's not 1).1 and that fixed the problem.