On Lopez Island T-Mobile is pretty much the only thing I been able to use. This is in the village area too, not just south end. And T-Mobile gets service on most of the ferry ride to Anacortes. When you are waiting in line for 90 minutes for a ferry ride in your car, cell service is really appreciated.
Counterpoint to the anecdata: I carry both TMobile (work) and AT&T (personal) phones most of the time. Often the only way to get any data response in a major metro is to tether the TMobile one to the AT&T phone - this is in a major metro area, and largely the reason I switched away from Google Fi (though it turns out Apple Watch cellular is so useful that Google Fi isn’t an option at all anymore).
Yes, at our place in Southern California my ATT service is much better then my T-Mobile service.