Hi @tolin ,
Try shining a torch at an angle close to the screen and check if you can detect any images at all when the phone is turned on. They will be very faint if they are there at all so trying this in a darkened room may help.
If there are images it is a backlight problem. If there are no images to be seen, then either way as a DIY repair all you can do is to temporarily replace the LCD screen and check what happens and go from there.
''If still no good'' it is on the systemboard. You would need to find the schematics for the systemboard to locate the backlight power supply circuit. I searched online without result. You may have better luck.
''If it now works OK'' it was in the LCD display and the display cannot be repaired. It has to be replaced.
Here’s the ifixit [guide|123458] guide which may help with this
Replacement screens are available online. Just search for ''Huawei P20 Pro LCD screen'' to get results for suppliers.