But why would you need a custom modem for it to work? Can't you use a different modem and just get them to give you xDSL credentials? Login and password, enter VPI/VCI and plug it into the telephone line..? What is the "802.1X auth"?
No, you can't. It doesn't use a username/password for authentication, it uses a protocol known as 802.1X, which uses certificates (and the associated private key) that's stored on the device.