New phone wont connect to LTE

[Bishop_Realtalk]

I got a replacement phone from bestbuy because my last one was broken it was an att galaxy s4 I337. They gave me as a replacement an unlocked I337m and it wont connect to att lte nor will it update to 4.4

Is there a way to make it work or do i have to take it back and order another one?

 

[Bishop_Realtalk]

Anyone?

 

[Bishop_Realtalk]

Alright so I've solved one problem. I called AT&T and gave them the imei to one of my old GS4s and LTE works flawlessly however it won't update beyond 4.3
Does anyone have a solution to that?