Thanks for the tip Kalle, but strangely it do not work for me.
I did exactly the same before and now again on my Fairphone 5, bought with Murena pre-installed.
I have also tried setting MMS to compression from automatic, to the lowest, 100kB.
Now I also tried this trick to allow MMS using the mobile call connection.
I have Telenor network via the seller “virtual operator” ETCMobil.
It worked when i used “3” network Hallon (using “3” network), but they have bad geographic coverage.
In between, I used Telia via ETCMobil before they switched provider, Telia did work sometimes, bot mostly not and I did never see what was related: sending operator, where I was, size of attachment… all seemed random, and worked less end less. Now with Telenor, never.
Now I found this that may be handy for someone for Fairphone 6, their OS, Telia, Manually create a separate APN for MMS.
On a sidenote, my spare phone Sony Experia XA2 running Jolla Sailfish have MMS problem with same providers (I have not tried to fix it there), but that SMS/MMS messaging application is much better in that it for failed MMS send or recieve it sounds and displays error messages telling download of MMS failed, and also keep a placeholder for that in the message thread, so then I can easily ask sender to provide the picture by other means. Murena SMS/MMS app stupidly hides the problem 
And yes that REALLY should get fixed too.