If it's ATT, then it's likely DSL? If so, you'll have to go into the router settings and correct for PPOE and the loginID and password before your router will be allowed to connect to the DSL service.
I had same problem with on of my friends I hooked up the other day.
need to know:
1. DSL or cable service thru ATT?
2. Make and brand of Router
3. Operating System
4. Using wired or wireless connection...