I can't be certain here that the network configuration you've supplied will allow nginx to connect to audiobookshelf. If I were you I'd create a named docker network and connect both npm and audiobookshelf to it as described in their best practice documentation: https://nginxproxymanager.com/advanced-config/#best-practice-use-a-docker-network
A 503 indicates that NPM can't see the service you're trying to proxy to.
default
in this example is the actual name of the network, in this case we're modifying the default network.external: true
declares that the network has already been created,name: scoobydoo
is a way of customizing the name of the default network.