Newly installed screen wont work.

My father-in-law's Galaxy A12 screen cracked and I gave it a go to replace it. I have successfully done it a couple of times on iPhones. I ordered a new screen (without frame) and I followed all steps in this video (https://www.youtube.com/watch?v=ExGSpQZv...). Afterwards it would not light up although I felt the start up vibration. Unfortunately, I had not tested the screen by simply connecting it and starting the phone to rule out a faulty screen at delivery, although when I reconnected the old, cracked screen it did lit up at start-up so obviously there was no major hardware fault causing the black screen.

I ordered a new screen replacement and this time I "hardly touched it" before simply connecting the two screen cables, reconnecting the battery cable and the "power on button cable" and started the phone, but no picture. I connected the phone to my Macbook which found the phone and automatically started the Android File Transfer app. Also, I could hear chimes like messages dropping in. So, obviously, the phone works but neither of the two new screen replacements lit up, they are completely black.

What am I missing? Could they both be faulty at delivery? Could I have trashed them both when handling them? It should be simple: when connecting the new screens to the phone – no picture – but when I connect the old, cracked screen in the exact same way – light (no picture since the screen is all cracked).

(Also, there is blue protective plastic behind the screen cables, under the plastic there is glue. When I remove the plastic, the cables can be stuck to the back of the screen. Is this essential for it to work? Or is it just for fixing the cable and minimizing movement?)