Vehicle purchased, lost papers by mail – how to register without papers?

Hello, I bought a car from a dealer a few days ago – a used car (import from Spain). At the time of purchase the letter was still on the way from Spain to the dealer. The dealer assured me that the letter was there after 2-3 days at the latest, and he sent it back (“this happens already, don’t worry”. So long (until today) I was able to drive with a short-term license plate. Unfortunately, today it turned out that the papers are undetectable! What can I do? How can I register the car without papers? What do I need to do? Ask for help, I’m helpless.