The problem is that when I connect the motherboard through the 5V DC input (from the pc) the display works fine, but when I connect it through 12V from power brick the display backlight is lit but no text can be seen. What may be the problem? I have MKS Base v1.5 Marlin

It’s probably some trouble with the power source you have connected as the latter is not compatible with the specifications of your 3D printer. Check this out with the help of the user manual you obtained with the equipment.