Black screen after replacing screen

I have purchased an iPhone 7 Plus that was faulty due to the screen, the seller stated they tried to repair themselves but could only see a light behind the screen and not any display appearing on the screen. I have looked at the phone when it arrived and on the screen it had a red sort of plastic so I took that off, attached all the cables back in but left the covers off

I have then tried powering on but I just get a black screen, even when charging I do not get a charger icon.

Anyone have any ideas? Thanks