Seems that you might need to add a route between the main office to branch.
You can do this by adding/configuring a routing protocol (RIP, OSPF, etc) in both routers so they can exchange routes to the known networks between them.
Another solution (more easy) e to configure a static route from the main office network to the branch office.
The machines in the main office network need to know the route to reach the branch office.
Hope this is the case and that makes sense to you.
Post back if you need further assistance.