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.