I found an old thread (link at bottom) that was never fully answered but it seems I am having similar issues so I am hoping to hear if anyone else can help me?
To start off, my office is located in the heart of downtown Tampa, Florida where the coverage should be great and am using an iPhone 8 which I purchased from T-Mobile almost a year ago when I signed up. I had no issues whatsoever with coverage up until about a month ago. Outside of my building it still does appear to be working ok but now I am getting incredibly slow speeds inside my office (14th floor) and more specifically at my desk. I have filed numerous complaints only to be told the Engineers opened ticket, checked into this and everything is fine at the towers and with the network. Beyond this I have tried many many troubleshooting steps of my device both recommended by T-Mobile and whatever research I can do myself. My phone works flawlessly at home so I have a very hard time believing it is device specific. After all of this, I was recommended by T-Mobile Customer Service to try the signal booster (4G LTE Signal Booster) which was provided by T-Mobile at no charge.
I receive the strongest signal on my phone in the break room which has lots of windows. Also, speed tests performed in the break room yield best results (approx. 10Mbps download & 18Mbps upload). So this is where I placed the Window Unit and it is picking up full bars (all 5 lit up as well as the 4G LTE light on). I have placed the Coverage Unit at my desk and it displays a 5 for coverage area (should be sufficient). However, this seems to lead to little to no improvement on my phone at my desk. I am still only receiving 1 to 2 bars of LTE and when I run speed test I receive approx. 1 Mbps download & 3.5 Mbps upload (almost unusable). If I turn LTE off my phone shows full 4G signal strength and running speed test gets me approx. 4 Mbps download & 2 Mbps upload.
Another strange twist to the story is I have a co-worker who sits directly next to me. He has T-mobile plan with an Android phone made by LG which he also purchased from T-Mobile. His signal strength shows that he has full 4G LTE signal however he still gets the very slow speeds that I am getting when running speed tests. I really do not want to leave T-Mobile and everything seemed to be working just fine up until about a month ago. Any help would be greatly appreciated. Thanks, rm