Port Forwarding in router for remote connection

Revision 2
Hi:
My server is sql 2008 which is in my head office and i have developed a C# Windows application
I want to connect my office server from Branch using the application
i have port forwarded the router where sql server is installed, but when i connect from server from branch office it throws the following error

i have tried this using management studio also but i got the same error

a connection was successfully established with the server but then an error during login process. (Provider: TCP Provider, Error :0 - the specifed network is no longer available.) (Microsoft sql Server , error:64)