After some trouble working out the APN settings for MMS, I found a solution without having to change the message app to Hangouts. Hopefully, this will be useful to others too!
I used this guide:
https://www.helpforsmartphone.com/public/en/fairphone/fp2/android-5-1/guides/27/Set-up-MMS-Fairphone-FP2
…entering this information from Telenor’s customer service facebook page: https://www.facebook.com/telenornorge/posts/10153959411384803
And now it works fine.
Note: I only entered information in the yellow highlighted fields (as shown in the first link).