Hi,
You'll find that information in the Teltonika-Networks website, under RUTX11's Ordering section: https://teltonika-networks.com/product/rutx11/#ordering
I've also added the missing info to our Wiki: https://wiki.teltonika-networks.com/view/RUTX11_Supported_Frequency_Bands
To answer your question, no, the RUTX11 will not connect to AT&T. It has FCC and PTCRB, but certification for AT&T is still in progress. (You'll find certification info on our Wiki, here.)
When the operator certification is complete, the X11 will be ready for use in the AT&T network. I don't know the date, but when it happens it will probably be announced via a newsletter. If you wish to be informed of this (but also other news), you can subscribe to our newsletter from our website. Simply scroll down to the very bottom, enter your email and click 'Subscribe'.
Best of luck,
DM