How can I fix it's backlight

Hi Isaac,

Generally when there's a backlight failure, the problem is usually on the screen itself rather than the logic board, but both are possible. The actual light is integrated into the backlight/LCD/digitizer assembly and is generally not replaceable by itself; when there's an issue the cure is almost always to replace the entire display. Sadly iFixit doesn't have a guide for your phone to replacing the LCD assembly, but I did run across a Spanish site that has the one you'll need.

Manuales / Nokia 7.1 / screen

It is theoretically possible a backlight issue could be a fault on the motherboard, but since that's generally not repairable by us home DIY-ers, replacing the screen is the most reasonable course of action for you to take, especially when you can buy a new screen for under $20 USD on places like AliExpress, Amazon or eBay. Here's one of the first ones to come up on a search; they seem to be readily available.

For Nokia 7.1 Lcd Display Touch Screen Digitizer Assembly X7 8.1 For Nokia 7.1 Plus Lcd For Nokia 7 Screen Replacement Ta-1085 - Mobile Phone Lcd Screens - AliExpress

Good luck with your repair; let us know how it goes!