ATT used an Arris 'gateway' for their service and always told me it couldn't be bridged with NAT disabled to allow using a better router. I hate ATT and their crap. I started using EERO and in the setup, it asks if the modem is standalone or a combo unit with router and it shows 'No Problem' next to the area with combo unit- I tried it and it worked fine, so options exist.

One of the problems with bridging some combo units is that when an update is sent, it sometimes reverts some settings to their default.