PS - I also found this thread: Danish users of Fairphone E-OS - #26 by Bettina
where DK_murena_user writes this:
”You can make it work if you have the Brave browser installed and defined as standardbrowser and you have a MitID Nøgleviser. We are currently working on a hack to make the MitID app work and I will post if we succeed - but until then you can use the above version to sign in to all apps requiring the MitID sign in.”