Nginx container with multiple docker networks

Hi,
I got a server which is running multiple projects. One is smarthome, one is nextcloud, in future I maybe add git or something. I want all services exposed with https on port 443 and they should be distinguished over the hostname. They are on different subdomains. I am using nginx as a proxy.

I thought of how to encapsulate all projects and ended up with giving each project a different network so they can internally use the same ports. We have a network called cloud and one for smarthome…

If I would add an own nginx container for each project they would all use port 443 on the host and this would not work so I created one ‘god container’ with the whole nginx configuration (sure you could put all configs into a folder and link the configurations from the project folders; maybe later).
When I add one network it works: Port 80 will be redirected to 443; If it was accessed from a different hostname it gets rejected; If everything is fine, it opens the projects website.
When I add a network from another project it doesn’t work anymore and the websites just time out.

This is my conpose with two added networks, which doesnt work for me:

version: '3'
services:
  nginx:
    container_name: nginx
    image: nginx:stable
    volumes:
      - ./nginx.conf:/etc/nginx/nginx.conf
      - ./public.cer:/etc/ssl/public.cer
      - ./private.key:/etc/ssl/private.key
    environment:
      - TZ=Europe/Berlin
    restart: always
    networks:
      - smarthome
      - cloud
    ports:
      - "443:443"
      - "80:80"

networks:
  smarthome:
    external: true
  cloud:
    external: true

And this is my nginx config for one project. For more projects would copy the last part and change the server_name and proxy_pass accordingly:

events {

}

http {
    map $http_upgrade $connection_upgrade {
        default upgrade;
        '' close;
    }

    server {
        listen 80 default_server;
        server_name _;
        return 301 https://$host$request_uri;
    }

    server {
        listen                                    443 ssl default_server;
        ssl_certificate                           /etc/ssl/public.cer;
        ssl_certificate_key                       /etc/ssl/private.key;
        server_name _;
        return 404;
    }

    server {
        listen                                    443 ssl;
        server_name                               subdomain.domain.de;  
        ssl_certificate                           /etc/ssl/public.cer;
        ssl_certificate_key                       /etc/ssl/private.key;

        location / {
            proxy_pass                            http://containername:8123;
            proxy_set_header Host $host;
            proxy_http_version 1.1;
            proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
            proxy_set_header Upgrade $http_upgrade;
            proxy_set_header Connection $connection_upgrade;
        }
    }
}

Does somebody know how I can solve that problem?