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.

For the sake of correctness: T-Mobile CDMA frequencies were supported in San Francisco. But only with 3G speed according to the display. In practice, however, it was unusably slow (just enough for WhatsApp chats).
And I don't know whether T-Mobile also uses CDMA everywhere --> you need LTE, everything else is rubbish.

I travel frequently, does it work with SIM cards and in countries: Switzerland, Ukraine, Thailand and China?