Hi and welcome to the forum
Check to see if there is a #fairphoneangel near you that may be able to help isolate issues regarding the charger, cable, battery maybe even bottom module.
Whilst you do some basic analysis
- Try another charger and other cables
- Get a good magnifying glass and check the USB port for debris.
- Clean the USB port anyway. There can be fine dust that stops good contact and/or tarnishing of the contacts.
If that doesn’t help try dismantling the phone and removing the bottom module and check it’s contacts.
You can check the forum for disassembly guides and whilst doing so note the dampness indicators.
If your phone is under two years old you may still benefit from the warranty is there is no obvious damage, hence checking the dampness indicators.