Phone turns on but screen stays black after digitizer replacement

Hello,

I got an iPhone 5S for screen replacement. I removed the broken screen, plugged the new one to test if everything was working and it was. I transferred all the component from the old one to the new one and turned the iPhone ON and nothing. No logo. Tried other screens and same problem, even the broken one won't work.

I checked the connectors and everything is OK, no missing pin.

I tried a hard reset and nothing happened, no Apple logo.

I plugged the iPhone in iTunes and it recognizes the device, tried DFU mode, restore and it worked, but the screen still shows nothing.

I looked at the screen with a flashlight to see is it was the backlight that was defective but I see nothing at all.

I am out of clues, so if anyone for an answer for that one it would be appreciated.

Thank you.