[Users] Openser ALG setup

Hello All, I am setting up Openser in this configuration: Client ---------> (private interface) Router (public interface) --------> Registrar The router is configured with Openser having mediaproxy module support. What I want to achieve is for nat traversal to happen at the router side such that when the sip messages reach the registrar, it would already contain the public IP address in its contact header field. I am using Openser here more like an application level gateway. Openser is listening on the public interface only. Currently, my setup is not working with the private address of the client registering in the registrar. Is this a possible setup? Can someone show me the way? Thamk you very much. Hans