Since you did not provide enough information as to what exactly the problem is and what you have or haven't done I will put my two sense in. If it does not help you simply look over it.
It may be the fact that you haven't set up your server/ computer properly. Below are a few of the DNS websites that will allow you to use your computer as a host.
Also another tip that may help you is adding your DNS information into your host file within your computer.
C:\WINDOWS\system32\drivers\etc
Simple terms. Go to "My computer" > "Local Disk C" > "Windows" > "System32" > "Drivers" > "Etc" > Now open the "Hosts" file with notepad then add the following below.
(External IP) (host thingy such as "test-online.servegame")
(Internal IP) (host thingy such as "test-online.servegame")
Hmm that's about all of my two sense I will put in for now. Next time be exact on the issue your having.