still no display after replaced screen digitizer

I bought 2 Samsung Galaxy Note 3 smartphones, both with cracked screens and ordered a screen + digitizer(this one: https://goo.gl/mLvzAM) and replaced the screen according to this video(https://goo.gl/NwE3Mv) and all worked well during testing and left the phone on the whole day and night. However, when I should open the screen to use the phone there was no screen. I felt vibrations, the blue, red and green Led 's are functioning and if you look very good you see the apps and images through the dark screen, so there is not enough light to let the screen function.( dark screen - no backlight)

I couldn't figure out what it was so I used my other Note 3 to replace the screen, but this had the same result: everything works but a very dark display.(dark screen - no backlight)

So I contacted the seller to claim a new screen but this was out of the question: "Every screen is tested when they leave the factory, so the screen is 100%" he said. So I have to believe him I guess.....

So I used the (wellknown) alternative ways to try to get my Note 3 to live: full battery charge, battery change, battery removed for a day, factory reset, hard reset, (Volume up, home and power button pushed in at the same time) but nothing helps.

Who has the solution?