EDIT: It was a firewall issue. I disabled my firewall and it works.

https://listmonk.app/

The site loads properly on serverIP:5870 and if I change proxy_pass http://127.0.0.1:5870; to proxy_pass http://listmonk.mydomain.com:5870; then it will load on listmonk.mydomain.com:5870. But it gives the 502 error when I visit the site without the port.

If I set proxy_pass http://127.0.0.1:5870; and visit listmonk.mydomain.com:5870 I get:

The connection for this site is not secure
listmonk.mydomain.com sent an invalid response.
[Try running Windows Network Diagnostics](javascript:diagnoseErrors()).
ERR_SSL_PROTOCOL_ERROR

docker-compose.yml:

version: "3.7"

x-app-defaults: &app-defaults
  restart: unless-stopped
  image: listmonk/listmonk:latest
  ports:
    - "5870:9000"
  networks:
    - listmonk
  environment:
    - TZ=Etc/UTC

x-db-defaults: &db-defaults
  image: postgres:13
  ports:
    - "9432:5432"
  networks:
    - listmonk
  environment:
    - POSTGRES_PASSWORD=pw
    - POSTGRES_USER=listmonk
    - POSTGRES_DB=listmonk
  restart: unless-stopped
  healthcheck:
    test: ["CMD-SHELL", "pg_isready -U listmonk"]
    interval: 10s
    timeout: 5s
    retries: 6

services:
  db:
    <<: *db-defaults
    container_name: listmonk_db
    volumes:
      - type: volume
        source: listmonk-data
        target: /var/lib/postgresql/data

  app:
    <<: *app-defaults
    container_name: listmonk_app
    depends_on:
      - db
    volumes:
      - ./config.toml:/listmonk/config.toml
      - ./uploads:/listmonk/uploads

networks:
  listmonk:

volumes:
  listmonk-data:

nginx config:

server {
        listen              443 ssl;
        server_name            listmonk.example.com;

  location / {
     proxy_pass  http://127.0.0.1:5870;
     proxy_set_header   Host            $http_host;
     proxy_set_header   X-Real-IP       $remote_addr;
     proxy_set_header   X-Forwarded-For $proxy_add_x_forwarded_for; 
    }

}

server {
    listen              80;
    server_name            listmonk.example.com;
      location / {
return 301 https://$host$request_uri;
      }
}