Ahh, ok. That’s not DSL then. DSL comes through a twisted pair like your phone lines.
That’s cable. You might actually be able to get that modem working even if they don’t support it. That’s not exactly desirable because if you do need to call tech support, they might not help you if you have an unsupported modem.
I’m guessing the first step would be to register the MAC address (from the modem) with AT&T. The second step would probably be to set up any PPoE settings in the modem (if AT&T requires that on this service).
Is the service already ‘turned on’ at they location where you are trying to hook this modem up?
Typically you lease the modem from Comcast. Are you sure that you don’t have to give that back? They usually will hit you with a huge final bill if you don’t return all of the equipment.
If you are interested in trying to get this working just drop another post and I’ll see if I can grab a manual for your modem. In the meantime, if you would point me to AT&T’s instructions for getting an AT&T approved modem working, we could get started giving it our best shot.