This can be done by authenticating, then using a managed switch to swap the wall module from the VLAN with the at&t router, to a VLAN with another router spoofing the at&t router's MAC address.
I've been using this approach for years and have found it worth it as I do not trust the provided router. The downside is I have to spend five minutes going through this process every time there is a power outage, luckily there are only a few a year here so not much of an issue.