This is precisely the reason why despite being a T-Mobile customer, I always buy my iPhone directly from the Apple Store and chose the 'worldliest model' (model/version which supports the most frequencies/LTE bands).

Usually that's the Sprint model which Apple employees will always try to tell you is locked to Sprint even if you buy it directly from Apple. That's not the case. It's just their agreement with Sprint to say that.

In the case of the iPhone 7 & 7+ its model A1660 & A1661. Has all the same bands as the GSM model but also adds TD-SCDMA, TD-LTE & CDMA EV-DO.