Ah that's rough, but we can take some steps that may help. Have you used our coverage map to see what coverage you're expected to be getting? Is this only happening in buildings or even outdoors? I've seen some cases where using a Non-T-Mobile phone can be hit or miss with service, but I'm curious if this may be isolated to a certain area.
Just wanted to check in on the questions Mike had above and see if you'd found a solution!
Your phone has support for all the bands that matter currently. Band 66 is only really used at concert venues, stadiums , airports, etc. Band 71 is still in the early stages of being depend. Not sure where you are located so I can not say how much of a impact this will have on you. I would like to know how coverage is outside. You mentioned having poor service indoors, which is something that is typical of any carrier. I.e maybe the Verizon tower in that market was set up just right to service in that building.
It is also worth noting that any location that has a wifi connecting can be used as a extension of coverage using wifi calling. That will allow you to place phone calls, send texts, and browse the web per the usual means. I suggest checking the coverage map which was mentioned to get a better idea of what you should expect in certain areas with T-mobile.