Note 3 network issues

I have a note 3 sm-n900t that I just bought from a private party. He said it's unlocked so I bought it and haven't had any issues until I upgraded to lollipop. One I upgraded via Odin I was having issues so I found out a factory reset would fix most issues. So after a factory reset it seemed better but had no network and the phone kept telling me I needed to insert a t-mobile sim. Couldn't find anything on request unlocking so I downgraded back to 4.4.2 and magically my network was back. Not sure what this issue is but I've checked apns and network settings and finding nothing works. Any help would be appreciated, Thanks