What would cause the phone to get poor signal?

I have a LG G4 AT&T (H810/6782A) and the phone gets little to no signal in the house, but when I go outside to try and get signal I still have problems getting any sort of reasonable signal.

I have tried a spare phone I have (Lumia 635) that I keep as a spare phone (which I am using right now), and that gets signal both in the house and outside; I can even make a call without the signal breaking up. I can even make a call inside. I average 2 bars in the house and 3 outside with the Nokia, and has no problems with getting signal.

The LG doesn't get signal inside and struggles to get (and retain) a signal (which has not happened on the Nokia), so it's a phone problem.

I have taken the back off so I could look for anything obvious on the board, and I never seen anything that stuck out to me as a problem from a quick glance of the board. As a phone, it is useless because of this problem.

Is there anything I can do to fix it? This phone is currently on a payment plan and it can't be removed without paying the balance on the phone (though I suppose I can switch phones without this).