Sounds daft, but it's easy to forget the basics when focused on something new so I'll ask, did you remember to update you MAIN servers firewall rules to allow connections from the reverse proxy server IP address?
You also need to make sure your MAIN config has the port open for receiving http messages from nginx so if your http broadcast port was 8000 or 8080 for example, you will want to include that, but you will also want to include port 80 in the main config as that is the port nginx will use for http protocol.
Edit: Ignore above, I can see from above you have included port 80 in your config.