LCD screen Dim Backlight issue

Hi everyone, I'm a computer technician. A customer brought me a galaxy tablet (Galaxy tab A9 plus (SM-X210) complaining regarding very dark screen whilst the tablet still on but you can only see things on the screen under the sunlight or under UV light, which is still very hard to see! However I have replaced a brand new screen and resetting it to factory settings and updated but still the same. Now I know the issue is not the actual screen. I'm assuming the backlight for the LCD screen it is on the motherboard but I do not have the motherboard diagram for this model so I can desolder or replace. I'm not too sure where the component for the backlight is located or what is causing this. thank you in advance. Any help greatly appreciated.