FP support is clearing a backlog of support tickets now: Current Support Status (Last update 10.01.18)
I would suggest waiting some more days for a reply to your ticket, the average reply time is quoted as 12 days in that topic. Also, note that a backup battery is much cheaper than a backup phone, and can be bought at https://shop.fairphone.com/spareparts/batteryfp2.html. I agree though that you deserve a replacement under warranty rather than having to spend €20.16 + shipment for a battery.