We are loyal T Mobile customers for more than ten years with six active T Mobile lines. The two lines that are connected via T Mobile's 4G compatible Lumia 521 phones have very low or no data flow since 01/11/21. We have strong five bar signal strength, but 80% of the time, there is no data flow which would cause the google.com webpage to time-out after one minute. When there is data flow, the speed is 0.7 to 2.5 M upload, and 0.02 to 0.2 M download.
This is a 24/7 problem in Southern California (Pasadena, Diamond Bar, Huntington Beach, Alhambra, etc). I could fix this problem if I insert an AT&T MVNO SIM card in the unlocked T Mobile Lumia 521 phone.
Running speed test with the T Mobile SIM card in another unlocked 4G LTE capable phone yields 35-45 M download speed. Therefore, the slow data problem may be limited to the T Mobile 4G bands.
Data flow at T Mobile was stable prior to 01/11/21 with 6 to 9 M download, and 0.7-0.9 M upload using the 4G capable Lumia 521 phones. The speed can hit 17 M during off-peak periods.
Based on these tests, the root cause may be the T Mobile 4G band and/or the switching/processing of this 4G signal at the T Mobile central office.
I filed four different trouble incident reports with T Mobile. All of these reports were closed by T Mobile within 3 days blaming a local tower issue due to system upgrade, or no problem found with the network. I asked the T Mobile CSR to provide us with a temporary T Mobile Hotspot so that we can access the data that we paid for. The T Mobile CSR say that T Mobile does not have such a device, which is not true.
Has anyone experienced data problem with the T Mobile 4G bands (non-LTE)? If this problem is not resolved in two weeks, we will switch the phone service to AT&T towers. I suspect the network integration between T Mobile and Sprint is not going well, and T Mobile is not willing to spend some money on affected customers (free temporary T Mobile hot-spot device) to fix the problem.