Here are the steps you must take in order to establish Radmin connection over the Internet: (including behind a router)
1. Download Radmin Server 3.x here http://www.famatech.com/download/rserv31.exe and install it on the remote computer. A remote computer is a computer which you want to gain access to later via Radmin.
2. Download Radmin Viewer 3.x here http://www.famatech.com/download/rview31.exe and install it on the local computer. A local computer is a computer from which you connect to the remote computer.
3. If you are connecting over the Internet you must know the external IP address of the remote computer. To find it out, from the remote computer visit http://www.famatech.com/myip.php.
4. On the remote computer right click on the blue Radmin icon in the system tray and click "Settings for Radmin server". Click "Permissions", make sure Radmin security is selected and then click the corresponding "Permissions" button. Click "Add user", choose username and password and click OK, OK. Then exit Radmin Server settings. You are done with the settings of Radmin Server.
5. The port you need to use is 4899. This is a default port, you can change it if you want, in Radmin Server Settings -> Options.
6. If you have a broadband connection on the remote computer side, you need to set up port forwarding rule on your router. Please, read what port forwarding is here http://www.portforward.com/help/portforwarding.htm. After that, visit this page http://www.portforward.com/english/applications/port_forwarding/radmin_-_Remote_Administrator/radmin_-_Remote_Administratorindex.htm , find you router model and follow the onpage instructions.
7. Again on the remote computer make sure that Windows built-in firewall doesn't block Radmin connection. Open Start -> Control panel -> Windows firewall, go to Exceptions tab and add Radmin port 4899 as an exception. If you have a proprietary firewall (for example, ZoneAlarm or any other firewall software) refer to its documentation to find out how to create exception for a port.
8. On the local computer open Radmin Viewer, on the main menu click "Connection -> New connection". Enter the IP address of the remote computer into "IP address or DNS name" field. Enter a descriptive name of the connection and click OK.
9. Now in order to connect to the remote computer in Full Control mode, right click on the newly created connection, and click Full Control in the context menu.