How have you setup the modem and router? Is the modem in BRIDGE mode or DHCP mode? Normally, with all routers (and many netgears) I have the modem in BRIDGE mode and then do the login and authentication (PPPoE) on the ROUTER. There is a setting for login, where you will put in your ATT username and PW, and then it will authenticate at the ROUTER through the modem.

If the modem is already authenticating, and you are getting DHCP from it directly, without doing a login from the computer (if you connect it like this) then I would say, turn off ALL login and authentication on the ROUTER and put it into DHCP mode.

But, I find the whole package works much better with the modem in bridge mode and the ROUTER doing the PPPoE authenticaiton.