Hi @Niki68
If you had the old listing in "Deactivation" status, you would probably need to change that to "active" before the system will let you edit anything. I'm assuming you did that? If so, did you try to edit the registration number on both a laptop/computer and try doing it on the app as well?
If so, and nothing works, you could create a new listing. The reviews will stay with the old listing, but since it's been awhile since you hosted (3 yrs), guests would probably wonder why there was such a large gap in bookings anyway. Also, with a new listing you will get the temporary "New" boost in search results which is a good thing. The reviews will still stay on your profile, but will not be attached to the new listing.