I have my EdgeRouter performing this function currently.
EDIT: Nevermind, I have (bonded) VDSL running through a 5268AC so I don't think I'll be able to do it. If it was "normal" Ethernet it would be possible.
The first you can put the modem in 'DMZ Plus' mode which is the closest you'll get to a bridge mode. This is where you'll lose bandwidth but it's easier to set up.
The second, which I recommend, is to connect your router to the ONT directly, and use their modem as a client on your network. You have to set up some rules to hook up the 802.1X traffic but otherwise the att modem is no longer in the picture. I haven't lost any bandwidth and I can't imagine that att's provided cheapo box would be faster than an EdgeRouter.
Comcast will let you bring your own modem and plug it into the coaxial. I've heard google fiber will let your bring your own stuff.
DMZ plus is much different than setting up an EdgeRouter to forward the authentication to the gateway. You are in much more control of your network if you don't use DMZ plus.
If I had even close to a 1Gb symmetric link I wouldn't care too much if I lost a couple megabits here or there (especially in the name of security or privacy). If I had a 30mbit link that only had 1mbit uplink I'd be upset not to use the whole tube but complaining about 980mbits vs 1000 is just a waste of time.
I'll do some tests myself. To see what the bandwidth lose is once I get an EdgeRouter.