Unable to connect Nginx and backend container in my droplet

hello. I am running my web app constituted of

• nginx (installed directly in the droplet)
• django container (backend)
• Next.js container (frontend0. I deploy all in the droplet I bought a domain name and pointed it to my IP server adress.

My app is running well locally, After deployement, I am getting the famous 502 bad gateway. i have the following files /etc/nginx/sites-available/web.conf

###########################################  UPSTREAMS

#upstream django_upstream {
#  server django:8000;
#}


#upstream nextjs_upstream {
#  server frontend:3000;
#}

############################################  SERVER

server {
    listen 80;
    server_name domain.com www.domain.com.ca;
    #server_tokens off;

    location / {
       try_files $uri @proxy_frontend;
    }

    location @proxy_frontend {
        proxy_set_header X-Forwarded-Proto https;
        proxy_set_header X-Url-Scheme $scheme;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header Host $http_host;
        proxy_redirect off;
        proxy_pass http://localhost:3000;
        #proxy_pass   http://nextjs_upstream; <--- (tried this with upstrea. but not worked)
    }

    
    #----------------------      Backend      ------------------------------


    location /backend/api {
        try_files $uri @proxy_api;
    }

    location /backend/api/docs {
        try_files $uri @proxy_api;
    }

    location /admin {
        try_files $uri @proxy_api;
    }

    location @proxy_api {
        proxy_set_header X-Forwarded-Proto https;
        proxy_set_header X-Url-Scheme $scheme;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header Host $http_host;
        proxy_redirect off;
        proxy_pass http://localhost:8000;
        # proxy_pass   http://django_upstream; <--- (tried this with upstrea. but not worked)
    }

    location /django_static/ {
        alias /app/backend/staticfiles/;
    }

    location /django_media/ {
        alias /app/backend/mediafiles/;
    }


}

and my docker-compose file is

version: '3.9'

services:
  db:
    build:
      context: .
      dockerfile: ./docker/database/Dockerfile
    container_name: c-db
    restart: always
    volumes:
      - db:/var/lib/postgresql/data
    environment:
      - POSTGRES_DB=${DB_NAME}
      - POSTGRES_USER=${DB_USER}
      - POSTGRES_PASSWORD=${DB_PASS}

  django:
    hostname: django
    restart: always
    image: i-api-prod:django-prod
    container_name: c-api
    build:
      context: .
      dockerfile: ./docker/backend/Dockerfile
    volumes:
      - staticfiles:/app/backend/staticfiles
      - mediafiles:/app/backend/mediafiles
    environment:
      - DB_HOST=db
      - DB_NAME=${DB_NAME}
      - DB_USER=${DB_USER}
      - DB_PASS=${DB_PASS}
      - SECRET_KEY=${DJANGO_SECRET_KEY}
      - ALLOWED_HOSTS=${DJANGO_ALLOWED_HOSTS}
    expose:
      - 8000
    depends_on:
      - db
    # networks:
    #   - django-network

  frontend:
    hostname: frontend
    build:
      context: .
      dockerfile: ./docker/frontend/Dockerfile
    container_name: c-frontend
    expose:
      - 3000

    # volumes:
    #   - frontend_static_assets:/app/frontend/static_assets
    depends_on:
      - django

volumes:
  db:
  staticfiles:
  mediafiles:

The .env file for my project is

# Database variables

DB_NAME=dbname
DB_USER=rootuser
DB_PASS=changeme



DJANGO_SECRET_KEY=secretkey_for_production_environment


DJANGO_ALLOWED_HOSTS=localhost 0.0.0.0 


# Frontend variables

API_BASE_URL="http:0.0.0.0/backend"

When I check nginx status is active(running).

• When I tried to a request in the broweser and check the logs, I obtained the following error.

2022/12/31 13:44:51 [error] 42558#42558: *62 connect() failed (111: Unknown error) while connecting to upstream, client: 31.2.136.184, server: domain.com, request: "GET /2.FKbxrpmVH1CttsoTtQEudY?O8A=bMnAfQJ7939h5tlyVuH_Uog&uS=ulh.AJJYMMaSJctgUQWEwiIZ.D&RPFsTVxAu=XkD HTTP/1.1", upstream: "http://localhost:3000/2.FKbxrpmVH1CttsoTtQEudY?O8A=bMnAfQJ7939h5tlyVuH_Uog&uS=ulh.AJJYMMaSJctgUQWEwiIZ.D&RPFsTVxAu=XkD", host: "www.ws.k12.ny.us"

and the understand that the problem is not nginx, but the connection to my backend and frontend. But I don’t know how to fix it.