ATT is really weird about it. They apparently decided a few years back that they'd not allow third party devices on their network, and keep doubling down on it.

Some folks smarter than myself figured out how to mimic the ONT handshakes or some such that they could use their own devices. IIRC, they were pulling the certs from the ATT box. ATT then came in and installed new stuff at the station to not allow that anymore. I don't remember all the technical details, but that's what I found when I spent an hour or so researching how to get rid of their awful box. In short, you can't...and even if you manage to, it won't be for long.