iPhone 6 backlight is working but there's no image on the screen?

Hey everyone. First post here.

I've been trying my hand at Iphone repairs lately and I'm at a point where I'm sort of stumped.

I bought an Iphone 6 that was sold as a "dead Iphone". They said it sometimes showed the Itunes logo and the phone would intermittently connect to itunes. I replaced the lightning port with a known good one. The phone still isn't recognized by Itunes but when you plug the phone into a wall charger the backlight comes on but nothing else. I tried it out with a few different screens but no luck. Any ideas what might be up with this phone?

I have yet to try a new battery. Could this some how have something to do with the battery? I'm sort of grasping at straws here but I'm not sure what else it could be.

UPDATE

-Just tried to put the phone in dfu mode. The phones backlight lights up like it would when you put any other phone in dfu mode...so we can rule that one out.