It's not clear exactly where your problem is located. Any further information that you can provide would help us figure out your issue.

That tutorial assumes that you are running two separate servers, a proxy server and an app server. You could simplify the set up by running the Nginx reverse proxy on the same server as the Node app. Essentially, all you would need to change is instead of using APP_PRIVATE_IP_ADDRESS use 127.0.0.1 This will serve your Node app on localhost, and then Nginx will proxy it to port 80 where it will be publically available.

For instance, this is what your Nginx configuration would look like:

server {
    listen 80;

    server_name example.com;

    location / {
        proxy_pass http://127.0.0.1:8080;
        proxy_http_version 1.1;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection 'upgrade';
        proxy_set_header Host $host;
        proxy_cache_bypass $http_upgrade;
    }
}