I wanted to write something about that anyway ;-)
Does NOT work with this version! Tried it out on the US West Coast in May 2018.
AT&T sells over there (but IMHO only in the online store) a US version with other bands which is SIM-locked.
The (European?) version that Digitec sells here does not fit the AT&T or T-Mobile LTE bands in the US.
Swisscom in CH works fine with top speed.
Since this is a flagship model with a corresponding price (and supposedly the best modem on the market) I never thought of checking the frequencies. Today, every iPhone supports 20 or more LTE bands and works practically everywhere in the world. Unfortunately, this hotspot does not. A huge minus point.
It is also clunky and heavy (but the battery is great). Ethernet is nice but rather a niche feature.
The advertised functionality with use in the hotel/airport (offloading) does not work in practice because captive portals are not supported (and which hotel has a fixed WiFi password?). -> Almost none - not in the USA anyway).
Conclusion: For the USA, it will probably be something like this next time:
Huawei E5788U-96A
As far as I can see, the thing supports AT&T and T-Mobile LTE bands.
I hope the feedback helps you and you don't have to pay a lesson like I did.