Thanks for the additional info! Thankfully, it worked in ProtonPass 1st try and gave legit 2FA codes, without needing to change any further settings.
I was aware that some authenticators run into errors with Lemmy for one reason or another, but that was forunately not the case here.
You may already be comfortable with another app or the web version of Lemmy, but I recommend 'Connect for Lemmy' if you're looking for instance blocking. It allows you to block instances as a user.