Docker Network with Nginx Proxy Manager

[EyyupA]

I used this docker compose file:

version: '3.8'
services:
  app:
    image: 'jc21/nginx-proxy-manager:2.9.18'
    restart: unless-stopped
    ports:
      # These ports are in format <host-port>:<container-port>
      - '80:80' # Public HTTP Port
      - '443:443' # Public HTTPS Port
      #- '81:81' # Admin Web Port
      # Add any other Stream port you want to expose
      # - '21:21' # FTP
    environment:
      # Mysql/Maria connection parameters:
      DB_MARIADB_HOST: "db"
      DB_MARIADB_PORT: 3306
      DB_MARIADB_USER: "npm"
      DB_MARIADB_PASSWORD: "npm"
      DB_MARIADB_NAME: "npm"
      # Uncomment this if IPv6 is not enabled on your host
      # DISABLE_IPV6: 'true'
    volumes:
      - ./data:/data
      - ./letsencrypt:/etc/letsencrypt
    networks:
      default:
    depends_on:
      - db

  db:
    image: 'webhippie/mariadb:latest'
    restart: unless-stopped
    environment:
      MARIADB_ROOT_PASSWORD: 'npm'
      MARIADB_DATABASE: 'npm'
      MARIADB_USER: 'npm'
      MARIADB_PASSWORD: 'npm'
    volumes:
      - ./mysql:/var/lib/mysql
    networks:
      default:

networks:
  default:
    attachable: true
    driver: bridge
    ipam:
      driver: default
      config:
        - subnet: 10.51.0.0/16
          gateway: 10.51.0.1

If I add a container to this network (for example checkmk) i got a bad gateway.
For checkmk port 5000 is used.
But if i use a service which run on port 80 is works fine.

And i recognized also if i use the portainer network it work also with checkmk.

Screenshot from the nginx network which will be created with the compose file

Screenshort from the portainer network (with this is works but why?)