Hi Bala,
Do you have SBS Essentials/Standard that have the RD Gateway? I'm not sure if you're running SBS/Server or Windows 7/Desktop. The RD Gateway only comes with the former.
Usually in the router you have an option for a public port and private port. You can set the public port to whatever number you between 10,000-65,000. Then when you connect from outside the office you need to specify that port number. Usually you can just add a colon and the port number after the IP.
Example with IP 70.0.0.1 and port number 38512
e.g. Computer name: 70.0.0.1 :38512
However when using SSL/RD Gateway, 443 should be the public and private. Then you login via the gateway.