Hey there! Do you have the specific model number of this phone? Also, what is your ZIP code where you are trying to use this phone? Lastly, I know you said you flashed roms and kernals but are you now back to the stock rom and SW version?
Hi grannyapple - I saw over on frequencycheck.com there are multiple HTC 10 versions. As tmo_chris said, model will help to see if it has the correct radios bands. Over on GSMArena i see that the following bands are supported for the US version:
LTE band 1(2100), 2(1900), 3(1800), 4(1700/2100), 5(850), 7(2600), 12(700), 13(700), 17(700), 20(800), 28(700), 29(700), 30(2300)
To work with TMO network you need LTE Bands 2,4,12,66 or 71 - so you should get some LTE signal...
Just dropping in to see if you still need some help. Please give us a holler and let us know how it's going.
Have you checked the coverage map to see what kind of service to expect? Do you have another phone you can try in the effected location? Do you know anyone else with T-mobile?