How to connect to Node API docker container from Angular Nginx container

I am currently working on an angular app using Rest API (Express, Nodejs) and Postgresql. Everything worked well when hosted on my local machine. After testing, I moved the images to Ubuntu server so the app can be hosted on an external port. I am able to access the angular frontend using the https://server-external-ip:80 but when trying to login, Nginx is not connecting to NodeApi. Here is my docker-compose file:

version: '3.0' 

services:

  db:
    image: postgres:9.6-alpine
    environment:
      POSTGRES_DB: myDb
      POSTGRES_PASSWORD: myPwd

    ports:
    - 5432:5432
    restart: always
    volumes:
    - ./postgres-data:/var/lib/postgresql/data
    networks:
    - my-network

  backend: # name of the second service
   image: myId/mynodeapi
   ports:
   - 3000:3000
   environment:
    POSTGRES_DB: myDb
    POSTGRES_PASSWORD: myPwd
    POSTGRES_PORT: 5432
    POSTGRES_HOST: db
   depends_on:
   - db
   networks:
   - my-network

   command: bash -c "sleep 20 && node server.js"

  myapp:
    image: myId/myangularapp
    ports:
    - "80:80"
    depends_on:
    - backend
    networks:
    - my-network

networks:
    my-network:

I am not sure what the apiUrl should be? I have tried the following and nothing worked:

1. apiUrl: "http://backend:3000/api"
2. apiUrl: "http://server-external-ip:3000/api"
3. apiUrl: "http://server-internal-ip:3000/api"
4. apiUrl: "http://localhost:3000/api"