I am looking for the carrier best suited for the watch. I live in New England which overall has good coverage by all the major carriers. When I have looked at compatability it has shown that many of the frequencies are not compatible with US service. All the G2 service is gone. And I could not figure out what carrier had the best G3/G4 for the watch.
You Should go for TMobile, I have TMobile, it show 4G network inside the house and outside too all the time.
So T-Mobile told me they were not compatible any details setup advice
Sometimes the carriers say " not compatible " which basically means they know nothing about these watches so its easier to say this .
I have used Speed Talk Sim Cards in my watches since day one. They run on the T-Mobile network and I have never had any issues with this service. Plus they are inexpensive. I use the $5 a month, auto refill cards, available on Amazon which is probably adequate for most users.
Andy is correct but also generally what I have found, with several carriers is when they say not compatible they are referring more to you cannot add the watch to your account, as another device. And then utilize it on your plan, like one of the watches they (desperately want to) sell you.