How do I set WA_DB_HOSTNAME to external server address (aws rds) for wacore

Question

I am trying to setup a cloud architecture on aws where there is an rds box that holds wa_db, and seperate instances of master and coreapps running in their own environment. I'm having trouble setting the path to the db (hostname) for wacore to connect to.

My docker-compose looks like this:

version: '3'

volumes:
  whatsappMedia:
    driver: local
  mysqlData:
    driver: local

services:
  db:
    image: mysql:5.7.26
    restart: always
    environment:
      MYSQL_ROOT_PASSWORD: testpass
      MYSQL_USER: testuser
      MYSQL_PASSWORD: testpass
    expose:
        - "33060"
    ports:
        - "33060:3306"
    volumes:
     - mysqlData:/var/lib/mysql
    network_mode: bridge
  wacore:
    image: docker.whatsapp.biz/coreapp:v${WA_API_VERSION:?Run docker-compose with env var WA_API_VERSION (ex. WA_API_VERSION=2.33.3 docker-compose <command> <options>)}
    command: ["/opt/whatsapp/bin/wait_on_mysql.sh", "/opt/whatsapp/bin/launch_within_docker.sh"]
    volumes:
     - whatsappMedia:/usr/local/wamedia
    env_file:
      - db.env
    environment:
      # This is the version of the docker templates being used to run WhatsApp Business API
      WA_RUNNING_ENV_VERSION: v2.2.3
      ORCHESTRATION: DOCKER-COMPOSE
      WA_API_KEY: "sizanani_WA_API_KEY"
    depends_on:
      - "db"
    network_mode: bridge
    links:
      - db
  waweb:
    image: docker.whatsapp.biz/web:v${WA_API_VERSION:?Run docker-compose with env var WA_API_VERSION (ex. WA_API_VERSION=2.33.3 docker-compose <command> <options>)}
    command: ["/opt/whatsapp/bin/wait_on_mysql.sh", "/opt/whatsapp/bin/launch_within_docker.sh"]
    ports:
     - "9090:443"
    volumes:
     - whatsappMedia:/usr/local/wamedia
    env_file:
      - db.env
    environment:
      WACORE_HOSTNAME: wacore
      # This is the version of the docker templates being used to run WhatsApp Business API
      WA_RUNNING_ENV_VERSION: v2.2.3
      ORCHESTRATION: DOCKER-COMPOSE
    depends_on:
      - "db"
      - "wacore"
    links:
      - db
      - wacore
    network_mode: bridge

i.e the default docker-compose for the dev-single-instance. Found under "installation".

My db.env looks like this.

WA_DB_ENGINE=MYSQL
WA_DB_HOSTNAME=db
WA_DB_PORT=3306
WA_DB_USERNAME=root
WA_DB_PASSWORD=testpass
WA_DB_CONNECTION_IDLE_TIMEOUT=180000

I would like to point WA_DB_HOSTNAME to a remote db, which seems possible according to their setup guide. However, I have tried WA_DB_HOSTNAME=localhost and WA_DB_HOSTNAME=127.0.0.1 to no avail. I have seen Weiyan Wang's answer which mentions that I do not need to access the db directly, however if the db is in a remote location I'd still need to update WA_DB_HOSTNAME.