Hi Looks like an issue with the phone contact support immediately via support@fairphone.com to notify of the problem as you have 14 days from receipt to send it back for a refund with no explanation after that only a replacement etc.
I use a similar charger 12V to 5V converter and 2.4A output, and had a similar problem as you initially but now charges at about 1% per 3 min. Please see my topic on this issue.