The default URL in /var/www/moodle/web/config.php was http://localhost/moodle and it didn’t work when I tried to access the application from a host other than localhost itself. So it was necessary to reconfigure the default URL to match the FQDN of the host which is accessible from remote hosts. In my testing infrastructure the HTTP service is accessible using the server FQDN from both red and green networks.
Another issue to note here is that the URL in config.php must be specified using https:// and not http://. Otherwise CSS and Javascripts aren’t loaded. In this case the application doesn’t work as expected. It seems the URL in config.php must match the schema used to call the application in first place. In our case, the one set by Moodle Box at NethServer Applications.
I guess the default Moodle configuration (as distributed by EPEL) was conceived to be prepared in localhost by the system administrator and later customized (by the system administrator as well) so it can be served in a public address when ready. Note the fact that, initially, the /etc/httpd/conf.d/moodle.conf file doesn’t allow access from hosts other than localhost and we had to change it in order to do so.