my experience

1) reset the Linksys router (read the manual how to reset)
2) reset the PC's (will get new IP from the router)
3) see if machices get IP from the DHCP of the router
(not the IP from the AT&T) by cmd IPCONFIG /ALL
4) configure the router with one of your PC's mac address (copy mac address from work station)

ps: most of the ISPs will not using PPPoe any more, check AT&T what they are using.

good luck