Using a secondary router behind the Surfwave Soho?

Hi. I want to use a secondary router (with DDWRT and VPN Configuration) behind the Surfwave Soho.

What changes do I have to make into the Soho to make it work?

So far this is what I got:
1 - Router B (secondary router with the VPN configuration) connected to router A (soho main router) through the WAN or LAN port = internet but no VPN connection.

2- Router B connected straight to the modem = Internet and VPN connection.

Any tips?