pymysql.err.OperationalError: (2003, "Can't connect to MySQL server on 'localhost' ([Errno 99] Cannot assign requested address)")

I have a streamlit app that requires connection to MariaDB. When I run docker-compose up I get this error:

pymysql.err.OperationalError: (2003, "Can't connect to MySQL server on 'localhost' ([Errno 99] Cannot assign requested address)")

I have a static IP where my app is hosted and an IP address for MariaDB and I am supposed to have access in this way.

This is my docker-compose.yaml file:

services:
    app:
        restart: always
        build: ./app
        ports:
            - "8501:8501"
        command: streamlit run Main.py

    mariadb:
        image: mariadb:10.5.17
        ports:
            - "3306:3306"
        volumes:
        - db_data:/var/lib/mysql
        - db_conf:/etc/mysql/conf.d
        environment:
            MARIADB_ROOT_PASSWORD: Ap$im
            MARIADB_HOST: xxx.xx.xxx.xx (IP address of MARIADB) 
            MARIADB_DATABASE: a
            MARIADB_USER: b
            MARIADB_PASSWORD: c
        networks:
            - streamlit_network

    nginx:
        restart: always
        build: ./nginx
        ports:
            - "80:80"
        depends_on:
            - app
            - mariadb

volumes:
    db_data:
    db_conf:

networks:
   streamlit_network:
     external: true

And the python file accessing it:

conn = pymysql.connect(
            host=os.environ.get("host"),
            user=os.environ.get("user"),
            password=os.environ.get("password"),
            database=os.environ.get("database"),
            cursorclass=pymysql.cursors.DictCursor

The .env file:

# Environment variables defined inside a .env file 
host="mariadb"
user="b"
password="c"
database="a"
port="3306"

Any help would be much appreciated. Thank you in advance.