Black screen on lockscreen but can still emergency call

OK so I’m repairing an iPhone 6 for a friend. They said that it had been broken a while but they never got round to getting it fixed. Being the iPhone enthusiast that I am, I offered to take a look for them. I’ve done quite a few repairs so I’m no stranger to diagnosing most issues. This iPhone seeming had a broken screen and my friend couldn’t recall any other problems before they stopped using it. The display was completely destroyed so I had to hook up a test display to test everything else. Having hooked up a known working display I was greeted with the usual charge battery symbol (I had put it on charge the day before but had no way to turn it off as the screen was unusable). Having put the phone on charge, it buzzed to say that it had enough power to boot and was now charging. However, the screen was completely black. My first thought was that the display connections weren’t seated properly so I reconnected the display. Still nothing. I then wondered if the connections were damaged in someway at board level. But then I noticed the VERY faint emergency call button at the bottom of the screen so I tapped it and the display lit up and slid across to the emergency call screen. However, when trying to return to the lockscreen the display goes black again after less than a second. Having done this several times I was able to read the message on screen telling me the iPhone was disabled - connect to iTunes. So after plugging it in to my PC and 2 different Macs with different lightning cables I get a small buzz from the phone but then it refuses to stay on charge or connect long enough to open iTunes. I have never seen something like this before and I can’t seem to find anything online about this issue either. The phone had previously been opened as it has a none genuine Apple battery fitted but it was clean and all of the shielding had been put back properly (which is usually an encouraging sign that it was done properly). I have no idea what to do next.

Please help!