Long time TMobile customer, and have been happy with the service until recently. About a month ago, I bought myself a Galaxy S10, and a week later my wife an iPhone XS. Both of these phones are less reliable on the network than our previous phones, especially at home.
On my Galaxy, I noticed that the data network would just stop working when at home and not connected to wifi. It would still show the LTE indicator and full signal strength as if it was connected properly, but the data network was not working - social media feeds stopped refreshing, web pages wouldn't load, and a speed test app would not connect to its server. Again, no indication on the phone at all that the network was down.
After some research, I downloaded the LTE Discovery app from the Play store and looked at it when the data network went down. It was always connected to LTE band 12. I used *#2263# to get into the advanced network settings and disabled band 12 on my phone, and have not had the problem since then. So there is definitely something wrong with band 12 on my Samsung or the tower near us. I re-enabled band 12 and got a better wifi router at home that would reach the whole house, so now I am on wifi at home and don't notice the problem. Yesterday, I turned off wifi to check if the band 12 issue is still there, and yes - within a few seconds, I could no longer refresh my social media feeds. Outside my home, I have no network problems, even with band 12 enabled - this is why I think there is a problem with band 12 on the tower near us.
When I got the iPhone XS for my wife, she started having similar issues with network connectivity, and even worse - phone calls were horrible quality, to the extent that I couldn't even talk to her for a few minutes without massive cutouts or the call dropping altogether. At this point, I called TMobile support, as something is clearly not right. They had me disable LTE for voice and data on the iPhone XS, and that seemed to fix both the data and voice problems. I told the rep about the band 12 issue I had found on my Samsung, but she didn't seem to understand what I was trying to convey. She did say that there was no indication of trouble on our tower that she could see, but they would have an engineer check it out. She also put in for new SIM cards for both phones, which we would receive in a few days (they didn't end up helping at all).
At this point, both phones were limping along well enough, so I figured I'd wait for the new SIM cards and try those. In the meantime, I figured that if they did investigate the tower, they would probably find and fix the issue and we'd be good to go. Well, here I am over a month later with no improvement. While my wife's iPhone works adequately at home, she has noticed that it is very unreliable outside the home - likely because at home she is typically on wifi, whereas outside she is not and has to rely on the non-LTE network technologies. She recently took a small road trip with our daughter, and found many instances where our daughter's phone (iPhone 8) had great coverage and my wife's iPhone XS was not usable. I had her enable LTE on the data again, since she can use wifi at home, but the voice is simply unusable at home with LTE enabled. Wifi calling, for whatever reason, is also not reliable at home - so that is not an option for voice.
So here I am, having spent almost $2000 on new phones that don't work properly. I am hesitant to call TMobile support again and have to tell this whole story, not knowing how much of it will actually be documented and investigated. I believe there are TMobile reps that frequent the forums, and I am hoping someone knowledgeable will pick this up and start it through the proper channels. Please help me get this issue resolved, it is becoming very frustrating.