Unable to connect to Postgres DB living in one docker-compose file with django app in separate docker-compose file

Question

I have a large monorepo Django app that I want to break into two separate repositories (1 to handle external api requests and the other to handle my front end that I plan on showing to users). I would still like to have both django apps have access to the same db when running things locally. Is there a way for me to do this? I'm running docker for both and am having issues with my front end facing django app being able to connect to the Postgres DB i have set up in a separate docker-compose file than the one I made for my front end app.

External API docker-compose file (Postgres DB docker image gets created here when running docker-compose up --build)

---
version: "3.9"
services:
  db:
    image: postgres:13.4
    ports:
      - "5432:5432"
    environment:
      - POSTGRES_DB=postgres
      - POSTGRES_USER=postgres
      - POSTGRES_PASSWORD=postgres

  api:
    restart: always
    build: .
    image: &img img-one
    command: bash start.sh
    volumes:
      - .:/app
    ports:
      - "8000:8000"
    depends_on:
      - db
    env_file:
      - variables.env

Front end facing docker-compose file (This is the one I want to be able to connect to the DB above):

---
version: "3.9"
services:
  dashboard:
    restart: always
    build: .
    image: &img img-two
    volumes:
      - .:/code
    ports:
      - "8010:8010"
    depends_on:
      - react-app
    env_file:
      - variables.env

  react-app:
    restart: always
    build: .
    image: *img
    command: yarn start
    env_file:
      - variables.env
    volumes:
      - .:/app
      - /app/node_modules
    ports:
      - "3050:3050"

Below is the database configuration I have set up in the front end django app that I want to connect to the DB but I keep getting connection refused errors when I try to run python manage.py runserver

DATABASES = {
    "default": {
        "ENGINE": "django.db.backends.postgresql",
        "NAME": os.environ.get("DB_NAME", "postgres"),
        "USER": os.environ.get("DB_USERNAME", "postgres"),
        "PASSWORD": os.environ.get("DB_PASSWORD", "postgres"),
        "HOST": os.environ.get("DB_HOSTNAME", "db"),
        "PORT": os.environ.get("DB_PORT", 5432),
    }
}

Any ideas on how to fix this issue? (For reference, I've also tried changing HOST to localhost instead of db but still get the same connection refused errors)