Postiz Self-Hosted - All working, but API access does not

[cmanblues]

Hi everybody! I have self-hosted Postiz on my server at home via Docker. I have it published to the Internet via NGINX Proxy Manager. All is working so far: I can register, login, connect channels, all good. But I cannot access the API. I have used the offical docker-compose from the Postiz docu and basically renamed 'localhost' with my URL. What have I done wrong?

services:
  postiz:
    image: ghcr.io/gitroomhq/postiz-app:latest
    container_name: postiz
    restart: always
    environment:
      # === Required Settings
      MAIN_URL: 'https://postiz.mydomain.com'
      FRONTEND_URL: 'https://postiz.mydomain.com'
      NEXT_PUBLIC_BACKEND_URL: 'https://postiz.mydomain.com/api'
      JWT_SECRET: 'bla bla bla'
      DATABASE_URL: 'postgresql://postiz-user:postiz-password@postiz-postgres:5432/postiz-db-local'
      REDIS_URL: 'redis://postiz-redis:6379'
      BACKEND_INTERNAL_URL: 'http://localhost:3000'
      TEMPORAL_ADDRESS: "temporal:7233"
      IS_GENERAL: 'true'
      DISABLE_REGISTRATION: 'true'
      RUN_CRON: 'true'
      NOT_SECURED: 'true'

When I open the API URL nothing happens for a few seconds, then it gets forwarded to https://postiz.mydomain.com:5000/api. 5000 being the internal port. When I open https://postiz.mydomain.com/api on the machine the docker container it is running on, I get "App is running", but not from other clients like iPhone or Mac Mini with Openclaw / Hermes in the same WiFi Network or over 5G.

What am I doing wrong...

Thank you,

Chris