Unable to connect to Mongodb container from flask container

#Docker compose file

version: "3.4"  # optional since v1.27.0
services:
  
  flaskblog:
    build:
      context: flaskblog
      dockerfile: Dockerfile
    
    image: flaskblog
    restart: unless-stopped
    environment:
      APP_ENV: "prod"
      APP_DEBUG: "False"
      APP_PORT: 5000
      MONGODB_DATABASE: flaskdb
      MONGODB_USERNAME: flaskuser
      MONGODB_PASSWORD: 
      MONGODB_HOSTNAME: mongodbuser
    volumes:
      - appdata:/var/www
    depends_on:
      - mongodb
    networks:
      - frontend
      - backend

  mongodb:
    image: mongo:4.2
    #container_name: mongodb
    restart: unless-stopped
    command: mongod --auth
    environment:
      MONGO_INITDB_ROOT_USERNAME: admin
      MONGO_INITDB_ROOT_PASSWORD: 
      MONGO_INITDB_DATABASE: flaskdb
      MONGODB_DATA_DIR: /data/db2
      MONDODB_LOG_DIR: /dev/null
    volumes:
      - mongodbdata:/data/db2
      #- ./mongo-init.js:/docker-entrypoint-initdb.d/mongo-init.js:ro
    networks:
      - backend
networks:
  
  backend:
    driver: bridge     

volumes:
  mongodbdata:
    driver: local
  appdata:
    driver: local
 

#Flask container file

FROM python:3.8-slim-buster
#python:3.6-stretch

LABEL MAINTAINER="Shekhar Banerjee"

ENV GROUP_ID=1000 \
    USER_ID=1000 \
    SECRET_KEY="4" \
    EMAIL_USER="dankml.com" \
    EMAIL_PASS="nzw"



RUN mkdir /home/logix3
WORKDIR /home/logix3
ADD . /home/logix3/

RUN pip install -r requirements.txt
RUN pip install gunicorn

RUN groupadd --gid $GROUP_ID www
RUN useradd --create-home -u $USER_ID --shell /bin/sh --gid www www

USER www

EXPOSE 5000/tcp


#CMD ["python","run.py"]


CMD [ "gunicorn", "-w", "4", "--bind", "127.0.0.1:5000", "run:app"]

These are my docker-compose file and Dockerfile for an application . The container for MongoDB and Flask are working fine individually, but when I try to run them together, I do not get any response on the localhost, There are no error messages in the logs or the containers

Curl in the system gives this :

$ curl -i http://127.0.0.1:5000 curl: (7) Failed to connect to 127.0.0.1 port 5000: Connection refused

Can anyone suggest how do I debug this ??

**Only backend network is functional

[
    {
        "Name": "mlspace_backend",
        "Id": "e7e37cad0058330c99c55ffea2b98281e0c6526763e34550db24431b30030b77",
        "Created": "2022-05-15T22:52:25.0281001+05:30",
        "Scope": "local",
        "Driver": "bridge",
        "EnableIPv6": false,
        "IPAM": {
            "Driver": "default",
            "Options": null,
            "Config": [
                {
                    "Subnet": "172.20.0.0/16",
                    "Gateway": "172.20.0.1"
                }
            ]
        },
        "Internal": false,
        "Attachable": true,
        "Ingress": false,
        "ConfigFrom": {
            "Network": ""
        },
        "ConfigOnly": false,
        "Containers": {
            "12c50f5f70d18b4c7ffc076177b59ff063a8ff81c4926c8cae0bf9e74dc2fc83": {
                "Name": "mlspace_mongodb_1",
                "EndpointID": "8278f672d9211aec9b539e14ae4eeea5e8f7aaef95448f44aab7ec1a8c61bb0b",
                "MacAddress": "02:42:ac:14:00:02",
                "IPv4Address": "172.20.0.2/16",
                "IPv6Address": ""
            },
            "75cde7a34d6b3c4517c536a06349579c6c39090a93a017b2c280d987701ed0cf": {
                "Name": "mlspace_flaskblog_1",
                "EndpointID": "20489de8841d937f01768170a89f74c37ed049d241b096c8de8424c51e58704c",
                "MacAddress": "02:42:ac:14:00:03",
                "IPv4Address": "172.20.0.3/16",
                "IPv6Address": ""
            }
        },
        "Options": {},
        "Labels": {
            "com.docker.compose.network": "backend",
            "com.docker.compose.project": "mlspace",
            "com.docker.compose.version": "1.23.2"
        }
    }
]

I checked the logs of the containers. no errors found, flask engine seem to be running but the site wont run , even curl won't given any output