I have ISP provided router on which upnp/natpmp is active and works correctly. There is a web form in which the registered clients/ports are displayed and it contains expected values in different usage scenarios.
I have introduced openwrt router between ISP's router and the rest of the internal network (lan). Everything works nicely (so far).
I am trying to set up upnp/natpmp "forwarding"/"cascading" or whatever it would be properly called (if it exists as a possibility).
What I need is for openwrt to:
 accept upnp/natpmp requests from the clients ad dynamically set its own firewall for forwarding the ports dynamically;
 request the dynamic port opening on the ISP's router (to act as a client at the same time).
Openwrt router currently properly registers request from the lan clients and updates the firewall.
Is it even possible to automatically act as a client for the same requested ports over its wan port?
I don't know if I have managed to explain this clearly enough. Please let me know.
Thanks a lot in advance!