Hi Leo and welcome to the forum.
Your original post has been moved here by a moderator.
This could just be a poor contact somewhere, battery or modules, in which case some cleaning might help. That sounds simple and basically it is, but if you’re not in the habit, it might be worth getting in touch with a Fairphone angel, if there’s one near you (see urs’s link above), or someone handy with such work.
There are risks in taking a phone to pieces, but with the phone in its present state it’s probably worth trying.
- For disassembly see the iFixit teardown here.
- Ground yourself before opening the phone (risk of static in some conditions).
- You’ll just need a good thumbnail or spudger and a Phillips #00 screwdriver. Put the screws aside in a small box or you’ll lose them.
- Never force anything, though moderate pressure is needed with the screwdriver to avoid stripping.
- Cleaning can be done using compressed air to remove dust, and then for the contacts, cotton buds soaked in isopropyl alcohol (isopropanol) or surgical grade alcohol (90°) available from pharmacies.
- After cleaning all contacts, including those of the battery and battery compartment, leave all items to dry in ambient temperature airflow for 3 hours before re-assembly.
- Be careful not to strip the screws when removing and replacing.