iPhone screen won't turn on to ISO only shows that the battery is dead

I recently got the phone to repair however its gonna a lot more complex then i originally thought it would be

The phone won’t post to ISO it will only show me that the battery needs charging then when the battery has charge the display goes black not even an apple logo. I thought it was then a logic board issue however the phone screen lit up a few seconds ago showing the home screen so now I’ve no idea what’s wrong with this phone.

From what i can tell the touch screen still works

The mute switch being toggled dose not make the phone vibrate so i’m still issuing a logic board error

If anyone can help please do and give me some instructions on how to diagnose logic board errors

It could also be a flex cable issue as the screen has started to light up when i apply slight pressure to the display cable

Thanks