Cannot connect from inside docker swarm cluster to external mongodb service

If I run single docker container of my backend, it runs well and connects to mongodb which is running on host. But when I run my backend using docker-compose, it doesn't connect to mongodb and prints to console:

MongoError: failed to connect to server [12.345.678.912:27017] on first connect [MongoError: connection 0 to 12.345.678.912:27017 timed out]

docker-compose.yml contents:

version: "3"
services:
  web:
    image: __BE-IMAGE__
    deploy:
      replicas: 1
      restart_policy:
        condition: on-failure
      resources:
        limits:
          cpus: "0.1"
          memory: 2048M
    ports:
      - "1337:8080"
    networks:
      - webnet
  visualizer:
    image: dockersamples/visualizer:stable
    ports:
      - "1340:8080"
    volumes:
      - "/var/run/docker.sock:/var/run/docker.sock"
    deploy:
      placement:
        constraints: [node.role == manager]
    networks:
      - webnet
networks:
  webnet:

how I run single docker container:

docker run -p 1337:8080 BE-IMAGE

Copyright Notice：Content Author:「Dmytro Nalyvaiko」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/44704450/cannot-connect-from-inside-docker-swarm-cluster-to-external-mongodb-service