There is a chance that when you get a new server for Call of Duty 4 that it won’t be listed on the master list and thus the public cannot join.  You can find out if your server has this problem by using the tool at the following website.

http://www.anarchic-x.com/cod4_server_audit/index.php

If your server fails the Master Server Position test then try to take your server offline and run the scan again.  If the message says the server is offline but still listed on the server list, then keep your server offline until it stops saying that your server is listed.  Then, you can restart your server and it should fix your problem.

If the above doesn’t fix your problem, the only thing I found on the internet is that changing your port can work.