Sounds like the network settings on the device could be wrong, or possibly your SIM card connection is failing/acting up (while radio failure is possible, but lets try not to consider that).
Try bringing your device to a T-Moble USA corporate store and they should be able to swap your SIM as well as verify the network settings on the device. If still no-go, they should be able to assist if you are under an active warranty.
I have been having the same problem and no one can help me! I have called Customer Care at least 8 times in the past month and a half! I got a new SIM card, still No Signal. I got a replacement phone, same problem. They told me to upgrade phone, I did that same problem. I received ANother replacement phone on Tuesday, SAME problem. Yesterday a customer service rep tried to help me, she said that my new SIM card wasn't registered on the network. She said she registered it and my phone should work now! NO SUCH LUCK!! The signal goes out, I can't get calls or make calls and forget about 4G! I have to turn off the phone and power back on and the signal is back for about 10 minutes, then its gone!! It is THE MOST FRUSTRATING thing ever!
I have tried everything - I do not know what else to do!! I am ready to cancel TMOBILE and go somewhere else! and its a shame because I have been a customer since 2007! But I haven't received any help and whats the point of paying a bill when I can't even use the phone!!
At wits end!