Just something that may or may not be helpful. If you look closely at the modem's setup, you might discover something like "service" or "hostname" that may be a key for the router to include - if it can. It may not be able to.
I'm several years away from playing with PPP but I recall using some PPP tools to get listings of the available services - if there's more than one it's a key to select the right one. Too often, routers use a strange name for this setting (like "put something here if your ISP says to"
It could be as simple as putting the right name as a hostname, too. Or NOT using what the router uses by default. Another setting obscured by simple router interfaces. The PPP negotiation needs a hostname and it may be generic or grabbed from the router's name, for example.
Like mentioned, AT&T forums may have the key. Good luck!
EDIT: Was just looking at »ui.linksys.com/files/E2500/1.0.0 ··· ppoe.htm
It's pretty clear where "Service Name (Optional)" goes. Host (and Domain) Name, too. I'd try every name you find in the modem.
Services can be a little harder to find.