Setting up SSL for custom port in nginx - letsencrypt
Let's encrypt http-01 challenges requires port 80 to exchange validation data. The https server is never used. Port 80 is a hard requirement. If that is not an option, then DNS is the only other way.
There is testservers that you should use until you have the setup correct (less rate limit, or maybe even no limit), first after that you switch to the production servers.
Similar question: https://community.letsencrypt.org/t/port-4434-instead-of-443/61349