Miggi

Both your containers need share at least one network. It looks like have created a external network. Good job. The next step is to configure your container to join the network. Do something like this:

services:
  nginx-proxy-manager: 
     ....
     networks:
       - nginx-proxy-manager_default # or just nginx
      ...

networks:
  nginx:
    name: nginx-proxy-manager_default
    external: true  

Don’t forget to also add the network in your audiobookshelf service.

Maybe this helps: https://docs.docker.com/compose/how-tos/networking/#use-an-existing-network

I have used some Hetzner products for some time now, including the Storage Box. The services they provide are great. My NAS backs up my important data to the Storage Box, and it just works fine. Sometimes I get emails that they need to do maintenance, but I never disturb my work. I think you can give it a try.