What I recommend is that you do the following procedure, it is a bit boring I know, but at least you’ll be able to have a greater certainty, and the staff are always very helpful.
Submit a support ticket (http://www.peplink.com/contact/support/) and provide the following information:
Your Country
Your Service Provider
Your Modem Manufacturer and Model
A Diagnostic Report file (Instructions)
Just in case anyone else finds this thread. The AT&T Momentum is a Sierra 313U, and works very well.
I had to do a firmware upgrade on the balance 20, download the Siearra watcher application and change the startup/shutdown option to do nothing ( see Peplink/Pepwave Router Compatibility Chart )
But, after doing that, I am now able to use the connection via the peplink balance 20 … and am very happy.