I can't follow the connection streams from the text description. If you can post a diagram or indicate the zones and connection paths we would be better able to help.
For example, What is the secondary office with a static public ip?
A VPN connection to the SSG
A router private connection and in what zone
A public address you want to connect to over the internet
Also you say "My firewall is configured without any policy restriction". But bear in mind that if there are no policies then the traffic is dropped not permitted. By default only traffic within the same zone is allowed. Any connection across zones is blocked unless you specifically allow it.