NGINX Reverse-Proxy for LXC Containers

Basic scenario:

I have a (physical) server running Proxmox.

On this hardware I run LXC containers (let's call them 100, 101...), all with NGINX started.

My goal:

Configure container 100 in a way that it acts as a reverse proxy to all my other containers.

Current configuration:

• on container 100 (the reverse proxy)

  server {
      listen 80 default_server;
      server_name _;
      location / {
              proxy_pass http://192.168.178.101:80;
      }
  }
  

• on container 101, one of the targets

  # This is a default site configuration which will simply return 404, preventing
  # chance access to any other virtualhost.
  
  server {
  listen 80 default_server;
  
  # Everything is a 404
  location / {
          return 404;
  }
  
  # You may need this to prevent return 404 recursion.
  location = /404.html {
          internal;
  }
  }
  

Both containers get a static IP, 100 get's 192.168.178.100 and 101 get's 192.168.178.101.

I can access 101 by it's IP from my browser, however 100 is unreachable.

It's the first time I'm setting up a network like this, pls excuse any stupid mistake I did!