Last year I joined T-Mobile. The coverage map shows this area is covered by extended range 5G, but the T-Mobile 5G hotspot I got would only get 4G no matter what was tried. Also leaving 5G enabled at all killed the 4G connection such that data completely dropped out for a second every 10 seconds.
I returned the hotspot and got a Samsung S22+ phone, since 4G data is throttled on everything but phones.
The Samsung S22+ had the same issue with extended range 5G, it wouldn't connect and I had to turn off 5G to get 4G working without the regular latency dropouts drops.
Both the hotspot and s22+ did work ok on other types of 5G, just not the extended range (band 71) available around my home.
Fast forward to this morning. I managed to open a hidden phone settings menu on the Samsung S22+ using a dialer code, and was messing around.
Suddenly the phone connected to extended range 5G with 5 bars of signal showing. It's been working on extended range 5G since then, even after resetting network settings and when travelling around the area in places covered by different extended range 5G towers. Data speeds are up by over 1000%
This appears to be strong evidence that the devices provided by T-Mobile and also factory unlocked devices are not configured properly to work on the T-Mobile network! Contacting T-Mobile customer support over the last year was useless... they'd tell me I was too far from the towers (but that's incorrect I'm getting 5 bars of 5G) and would tell me to reset network settings, reboot the phone, replace the sim card, and other stuff that did nothing to solve the problem.
Then something happened when using a samsung hidden menu to tweak the internal phone configuration that did fix the problem.
I hope this information helps T-Mobile serve their customers better.