Allow multiple ports to be accessible under HTTPS (ufw, nginx)

Question

I am right now fighting and reading documentations and articles about using the ufw firewall to allow multiple ports to be accessible via https.

I have tried enabling the desired ports using this command ufw allow port, but still not allowing any requests over https.

Deactivating ufw firewall does the job, but I am concerned about it, as I am not strong when it comes to servers and security, but believes the defaults are sufficient.

My nginx configuration looks like this:

server {
  listen 900 default_server ssl http2;
  listen [::]:900 default_server ssl http2;

  ...

  location / {
    proxy_pass http://application:9000;
    proxy_pass_request_headers on;
    proxy_set_header Host $host;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header X-Forwarded-Proto $scheme;
    proxy_redirect $scheme://$host:9000 $scheme://$host:80;
  }
}

And ufw status shows this:

To                         Action      From
--                         ------      ----
Nginx HTTP                 ALLOW       Anywhere
OpenSSH                    ALLOW       Anywhere
Nginx HTTPS                ALLOW       Anywhere
443/tcp                    ALLOW       Anywhere
80/tcp                     ALLOW       Anywhere
Nginx HTTP (v6)            ALLOW       Anywhere (v6)
OpenSSH (v6)               ALLOW       Anywhere (v6)
Nginx HTTPS (v6)           ALLOW       Anywhere (v6)
443/tcp (v6)               ALLOW       Anywhere (v6)
80/tcp (v6)                ALLOW       Anywhere (v6)