Last week I switched to T-Mobile from AT&T and I have an iPhone 6 (MG4W2LL/A). This past weekend I drove from Lansing, MI (home) to a cabin in the woods about three hours north near Luzerne, MI. When I was about an hour from the cabin I noticed that my phone was connected to the AT&T network, and it stayed on AT&T for the rest of the time I was in the area despite driving around to various small towns. The connection was always 1-4 bars of AT&T 4G. Data download speeds were dismal (~50KB max) and almost unusable, and I never saw LTE until I drove about an hour out of the area. I did a few speed tests using the OpenSignal app and most of the time I saw download speed errors, but interestingly ping times were steady in the 300ms range and upload speeds were in the 1-3 Mbps range. As a point of reference, at my home location my phone shows T-Mobile LTE service only and works perfectly in every way.
On T-Mobile's coverage map the entire area of my weekend travels is covered, and my specific location at the cabin is shown as T-Mobile 4G LTE (Fair.) However, I never saw the T-Mobile network at all on my phone so I'm thinking something is up. Is this a problem with the iPhone 6 lacking bands 12/66/71? Am I domestic roaming and going to face draconian data restrictions? Is my phone somehow still tied to AT&T in some way? I switched to T-Mobile from AT&T because I now need an "unlimited" data plan and I like some of the perks offered, but I'm wondering if I've made a mistake because I used to be able to receive LTE signal from AT&T at the cabin.