dmz = demilitarlied zone, meaning your router firewall lets everything through to that IP, you have your ISPconfig firewall so that should be fine. Also Make sure you shut down your firewall on Fedora.
To view your newly added sites enter the url of the newly added site. If you are trying to get to the control panel http://subscribe04.com:81
you can do it on the local machine or different machine. But you should have someone outside of your local network try to get to the site as well, as I think its your firewall blocking outside connections.
troubleshooting tips, try using your outside IP and see if you get the apache test page.
make sure port 80 and port 81 minimally are forwarded to your local IP 192.168.0.183