you can still get around this with some effort [1] and a pfsense box, the pfsense box gets wan from the ont and the original att router is hung off a third nic where it's allowed to do 802.1x and nothing else. the setup was a little challenging at first but has been maintenance free since. maybe there is a technical reason they have their network set up this way but i was offended at the idea of being prevented from using my own router.

[1] https://github.com/MonkWho/pfatt