iPhone not powering on after screen replacement, but original works?

I recently bought an iPhone 6 plus with a smashed screen on ebay, hoping to repair it with a replacement screen. I have done a few iphone screens before, and followed the same procedure, however when I turned on the phone I get no response, not recognised by itunes, no sound or vibration, nothing. I then replaced the smashed screen, as this one I could see when the phone was on but the touch did not work. The phone still powered on with the smashed screen on, so I hadn't damaged anything! I thought then the replacement screen must be faulty so I bought another one, but have the same issue. The phone was already on ios11 when I recieved it, and I have heard the update has caused some issues with replacement screens but was fixed in ios11.0.3 so updated via itunes, with the original smashed screen on, however both screens still do not work! Does anyone have any ideas?