Welcome to the community 
First things first, after the failed A12 update, did you let the phone bootloop a few times? 
If it’s unbootable it should have switched back to the other slot after a couple of failed booting attempts.
If you can still access recovery, you might be able to sideload an ota update (which Fairphone still doesn’t provide directly 
). If you’re on the latest A12, the next one’s probably this:
If the ota install works but it’s still unbootable, there are a few more updates you can try after that. You won’t be able to go up to the current release though, since nobody stepped up to post the newer ota links.
Another option to try would be a factory reset, but if your phone isn’t working at this point I’m gonna go ahead and recommend contactsupport.
There have been a few similar issues with previous Ubuntu Touch installations, I don’t remember the details, you’d have to search the forum for that.
For future reference, don’t ever lock anything when you’re going from an up-to-date system to a previous version. Locking the critical partitions doesn’t trigger rollback protection iirc, but there’s also absolutely no reason to lock the critical partitions at all.
If the bootloader is locked others don’t have access to them anyway, if you don’t intend to lock the bootloader then all you’re creating is a brick risk when you lock them.