[Q] ATT signal troubles on H815 US

I purchased my phone (h815 US model) online from swappa and I love the feel and look of the phone, Love the camera, BUT my phone wants to stay on hspa all the time. Where i had 3 or 4 bars on my s5 active I get HSPA unless I go in to settings and sellect ATT:LTE from network operators which only last until my phone goes to sleep then im back on hspa. Anyone else experiencing this? Or anything i should try? Thanks in advance for any help!