Are you using dyndns?
if yes, then it is possible that the services of dyndns in your router is not updated.
Please do the below steps:
1. Please type the URL http://www.checkip.com in the browser of the server
2. in this page you can see your current Public IP from your ISP. Please try to use this ip to replace your host name in the remote descktop client
3. if you are able to connect, it means the services of dyndns in your router is not updated.
You could install the Dyn updater software on a host computer running on your network and have that update your hostname(s). This would require that you have a computer running on your network 24/7. You can download and find information about installing the Dyn Updater here: http://dyn.com/support/clients/