Is the router's DHCP function enabled? Is that notebook set to acquire its IP/DNS/etc info automatically? If yes, and the Denon isn't able to get an IP address when it's set to it DHCP mode, then that's not a good sign. You may have some sort of compatibility issue.
I hate those combo DSL/router things. I am going to guess that's causing the problem. See if AT&T has any firmware updates for it.
You may have to go buy a 'real' router manufactured by a more mainstream company, like a D-Link or Linksys. Then you'd use the combo doodad as a Modem only, and use the new router to actually handle the routing/AP duties.
