Since the admin panel for this app is coming out of port 81 one of the ways you can connect to it is by ssh port forwarding.

From your local pc you open a terminal and write the following accordingly:
ssh -L 8081:yourhostip:81 user@host

You can always setup wireguard on the server and be done with it but the above solution is way faster for some initial setups.

Default Admin User:

Email: admin@example.com

Password: changeme

docker-compose.yml

---
services:
  app:
    image: 'jc21/nginx-proxy-manager:latest'
    restart: unless-stopped
    ports:
      - '80:80'
      - '81:81'
      - '443:443'
    environment:
      DB_MYSQL_HOST: "db"
      DB_MYSQL_PORT: 3306
      DB_MYSQL_USER: "**your_user**"
      DB_MYSQL_PASSWORD: "**your_db_password**"
      DB_MYSQL_NAME: "nginxproxy"
    volumes:
      - ./data:/data
      - ./letsencrypt:/etc/letsencrypt
  db:
    image: 'jc21/mariadb-aria:latest'
    restart: unless-stopped
    environment:
      MYSQL_ROOT_PASSWORD: '**your_mysql_root_password**'
      MYSQL_DATABASE: 'nginxproxy'
      MYSQL_USER: '**your_user**'
      MYSQL_PASSWORD: '**your_db_password**'
    volumes:
      - ./data/mysql:/var/lib/mysql