It’s been a while since you posted, but in case you haven’t contacted Fairphone support yet, I’d suggest to do that (though you could try a battery kickstart (under ‘charger’ first). What you’re describing sounds like a defective bottom module - Fairphone support has generally been replacing these under (the 2 year) warranty, but you’ll have to get in touch with them to get this sorted.
If you have any more questions about this issue, check the topic below and continue there (I’ll close this topic as the other topic is a better fit):