Best option is an outboard VPN router. That addresses security issues presented when port forwarding.
In many, but perhaps not all AT&T regions, no instructions, even those provided by AT&T themselves, to port forward on the Pace 5268C (they call the process "opening pinholes") or put it in 'Bridge mode' in order to use an outboard router...will work.
You can spend fruitless hours speaking with AT&T support or reading pages and pages of 'solutions' on various online forums and get no further than you are now.
I won't bore anyone with how I got port forwarding to work on an AT&T uVerse with a Pace 5268AC locally (I used an outboard router) because it very likely won't work in YOUR area.
Don't ya just love AT&T? Who else would change a term like "port forwarding", which works just fine and is not broken, to "opening pinholes"? Or provide a residential 4G/LTE cellular router/hot spot with configurable features like "port forwarding", "DMZ" and "VPN Pass-thru", none of which work?
I cannot believe the SEC allowed them to gobble up DirecTV. What's next....Sirius/XM? Or Bank of America? GM?
I feel so much better now. Well, not really.