From the user guide[1], it appears to be a dual-WAN setup that uses both cellular and broadband. Not as good as decent POTS, but better than just cellular or broadband.

https://www.att.com/scmsassets/support/other/att-phone-advan...

My understanding is it's wireless first, with failover to broadband. The broadband connection is optional.

They cannot manage the QoS over the broadband link.

My personal experience is any power outage lasting more than a few hours is also going to take out your community's broadband.