docker compose connecting mongodb container with node js container

i have a sharelatex container running. This docker compose file contains a mongo and redis container.

Here is the sharelatex docker compose:

version: '2'
services:
    sharelatex:
        restart: always
        image: rigon/sharelatex-full
        #image: sharelatex/sharelatex
        container_name: sharelatex
        networks:
            - test-network
        depends_on:
            - mongo
            - redis
        privileged: true
        links:
            - mongo
            - redis
        volumes:
            - ./sharelatex_data:/var/lib/sharelatex
            - /var/run/docker.sock:/var/run/docker.sock
        environment:
            SHARELATEX_MONGO_URL: mongodb://mongo/sharelatex
            SHARELATEX_REDIS_HOST: redis
            SHARELATEX_APP_NAME: ShareLaTeX
            SHARELATEX_SITE_URL: https://latex.tkwant.de

   mongo:
        restart: always
        image: mongo
        container_name: mongo

        expose:
           - 27017
        volumes:
            - ./mongo_data:/data/db
        networks:
            - test-network
    redis:
        restart: always
        image: redis
        container_name: redis

        networks:
            - test-network
        expose:
            - 6379
        volumes:
            - ./redis_data:/data

networks:
    test-network:
      external: true

I want to create a node application which needs mongodb, too. How can i connect these two container? I read about network and tried out docker network but without success.

This is my node docker compose:

version: '3.5'
services:
  app:
    container_name: app
    restart: always
    build: .
    ports:
      - '3001:3000'
    networks:
      - test-network
networks:
    test-network:
      driver: external

and here my index.js:

// Connect to MongoDB
mongoose
  .connect(
    'mongodb://mongo:27017/test2',
    { useNewUrlParser: true }
  )
  .then(() => console.log('MongoDB Connected'))
  .catch(err => console.log("errorErrorError"));

I am open for all answers... running mongo in an own container or create a docker network. But I dont know what is the best or the easiest.

Update 1:

First Problem: Sharelatex does not work anymore --> sharelatex is now in another network. My nginx reverse proxy does not find sharelatex container anymore. Second Problem is: when i want to start node docker i get this error (dont know why but it want to create a new network): Creating network "dockernodemongo_test-network" with driver "external" ERROR: plugin "external" not found

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