Multiple Battery/LCD combinations won't work.

Hi all!

Long story:

My parents sent me an iPhone 6 my mother used to use. The screen was cracked, the LCD was having issues, and the battery was acting up. When I first plugged the phone in to charge I was left on the battery symbol against the black background with a sliver of red for 15+ minutes indicating it was charging, but I don't feel as though it really was. When unplugged, the screen displayed a charging cord underneath this battery, obviously indicating it needed to be plugged in. I figured a new batter would fix this issue but to no avail, now that the battery and screen are both replaced, I am no longer getting anything on the screen. Multiple battery / LCD combinations have given me the same standstill of the phone not turning on or showing any signs of life. Plugging in the charger in any of the scenarios yielded no results either. What could be the issue and how can I fix it?

Long story short:

1. iPhone 6 had Screen issues and Battery issues.

2. When plugged in for any length of time, battery did not seem to be charging enough to turn on before repair.

3. After repair, no response on screen at all, even when plugged in.

4. No screen / battery combinations have seemed to be working.

5. What do?

P.S. Yes, I know to unplug the battery before unplugging the screen for repair, and the parts were ordered from my store's (iPhone repair store) distributor as premium parts.