Unlocked, working, but then show "NO SERVICE". LTE Ok, but no signal, no phone calls. topic

Maybe you guys from international forum can help me find a solution for this.

Here is the scenario:
I bought the phone from a Swappa Seller. Phone is t-mobile branded and was in the box, Brand new.
http://swappa.com/listing/ZPB505/view

Quote:

Brand New Samsung Galaxy Note 4 SM-N910T White ( UNLOCKED) 32GB Phone is brand new in its original box. The phone has never been used. All Accessories are still sealed in plastic - The phone model is SM-N910T. This is a great deal on a BRAND new - NEVER USED phone! This phone is guaranteed to work with AT&T. There is no regional lock whatsoever

Quote:

Achraf G. Seller - Nov 03, 2014 - 11:32 PM You will have to add for one day-shipping. Let me know if that works for you. Thanks

Can anybody tell me why a T-mobile unlocked phone won't work with T-mobile?

I used the phone on my carrier and it was flawless. Everything was fine till suddenly i lost phone signal. It always show "NO SERVICE" message now, but strangely I still have 4G Connection. I can use the phone to navigate, not to make phone calls.
So I swapped carriers, and the phone was working ok, phone calls, data connection.... then the thing happened again: no service, but i have 4g LTE connection.

Inserted a third sim card, from another carrier. Phone is fine, workin as it should! But this is not my main sim card and i would like it very much to work with ANY carrier. Do you guys have any idea what the problem is? Factory reset did not work at all. For the others sim cards, it recognize the network for a second and then "no service". How can i have data connection if there is "no service"?

Im really desperate because i payed a lot for this note 4... Maybe if i root the phone and flash another ROM?

help please!