Did you check TMobile's signal in your location with a TMobile phone before you did this? (If they have poor coverage there, it's not the phone's fault.)
Otherwise, I'd take the phone into TMobile and ask for assistance. (The card could be bad, your account provisioning could be bad, etc.)
BTW, it has nothing to do with how it was unlocked. If it doesn't ask for an unlock code with the TMobile SIM, it's correctly unlocked.