Why Cant I connect to the Mobile network?

[AC Question]

I have a Samsung Galaxy S6 Edge. My wife and I bought the same phones at the same time. We connected it to a network where it worked fine for at least 6 months. One morning, I looked at my phone and it said the sim was locked and I was not connected to any network. At this time for the first time a Security Alert This phone has been flashed with an unauthorized software... and continues on to say contact my phone provider. That day I decided to switch my phone carriers to T-Mobile. When I did, the locked sim issue was solved but I can not connect to their network. I can make calls via internet, and recieve them using my phone number as long as I am connected to wifi but not on the T-Mobile network. I have taken the phone to the store and they see nothing wrong with it. I have taken it to Best Buy, and they see nothing wrong with it. I have talked to T-Mobile support, and they can find nothing wrong with it. I have used all types of problem solving solutions I have found on the internet with no luck. when I press *#60# or whatever it is, I get a good imei.
in "About Device" I have a good Baseband Version that does not say "unknown".
I assume I have the updated software because when I go to the software it says last checked 7/15/17 (Today) but when I click it, it wont download anything.
I took it to Best Buy and had the OS flashed (still get that alert when the phone comes on)
I did 2 factory reset.

HELP!!!

I assume this has been tampered with in the past (unlocked or whatever) because I was able to be on my old network for the last 6 months even thought it's a T-Mobile phone. So how do I fix it if it was?

 

[B. Diddy]

Welcome to Android Central! What's the exact model number of the phone?