docker can't connect to another docker on same machine

Question

i have database in docker on my server, i write spring boot App that connect to that database from my local computer sucessfull, however when i was trying to containerize my app with docker in same server, app cannot connect to my database. in both cases i using my server public IP.

here is my database docker-compose:

version: '3.8'
services:
  db:
    image: postgres:14.1-alpine
    restart: always
    environment:
      - POSTGRES_USER=xxx
      - POSTGRES_PASSWORD=xxx
    ports:
      - '5432:5432'
    volumes:
      - db:/var/lib/postgresql/data
  pgadmin:
    container_name: pgadmin4_container
    image: dpage/pgadmin4
    restart: always
    environment:
      PGADMIN_DEFAULT_EMAIL: xxx
      PGADMIN_DEFAULT_PASSWORD: xxx
    ports:
      - "5050:80"
volumes:
  db:
    driver: local

and my app docker-compose:

version: '3'
services:
  backend:
    build: .
    ports:
      - 9090:9090
    environment:
      - SPRING_DATASOURCE_URL=jdbc:postgresql://XXX_IP:5432/db_name
      - SPRING_DATASOURCE_USERNAME=XXX
      - SPRING_DATASOURCE_PASSWORD=XXX
      - DRIVER_CLASS_NAME=org.postgresql.Driver

and if it matters my app Dockerfile:

FROM openjdk:19-jdk-alpine
ARG JAR_FILE=*.jar
COPY ${JAR_FILE} app.jar
ENTRYPOINT ["java","-jar","/app.jar"]

i aslo trying link my db docker compose in app compose with something like that:

version: '3'
services:
  backend:
    build: .
    ports:
      - 9090:9090
    links:
      - db
    environment:
      - SPRING_DATASOURCE_URL=jdbc:postgresql://db:5432/io_projekt
      - SPRING_DATASOURCE_USERNAME=xxx
      - SPRING_DATASOURCE_PASSWORD=xxx
      - DRIVER_CLASS_NAME=org.postgresql.Driver

but but it caused an error: no such service: db.

so ther is my question: what i doing wrong, and what can i do to connect my app docker to database docker?