If the server is on a company domain you can connect to that domain by VPN and then use rdp to manage the server. If it is not in a company domain then please give us details as to how everything is configured with this server. Thanks.
As another possibility our organization publishes a citrix portal page with rdp as one application. I can log in using that portal from anywhere I have internet connectivity. But if your organization does not use citrix then that is not a possibility for you.
Try LogMeIN or ShowMyPC. Both have free solutions but in most cases some of your authentication information passes through a third-party. This may or may not be a risk to your organization but is an easy way to “securely” remotely access a host behind a firewall.