Trying to fix an unlocked AT&T g4

I am on T-Mobile. My old LG G4 broke and I bought an unlocked phone off the internet to replace it. The phone that came happened to be AT&T branded. It works with T-Mobile and I was able to set up a T-Mobile APN. The problem is that the Operating System won't update at all (stuck on Android 5.1) and that the phone runs very slowly (maybe as a result of the old Operating System.)

Any idea on how to fix it. I still have the old LG G4 and while its on its last legs I could probably back up some data off it, but I don't know if that would transfer the Operating System. Is 5.1 the last OS update for AT&T phones? Or is this because I'm not using the correct SIM Card?

Thanks. I do love this phone but its seeming like it may be time to change.