I understand that you need to assign a static IP address and subnet on your Slingbox PRO.
We recommend you to connect the Slingbox directly to the router. Once you establish this connection, the Slingbox will pick up the IP address from the DCHP protocol. You can run the Setup Assistant for Internet Viewing and use the Manual Setup in order to modify the settings as you wish. Make sure to follow these indications and let us know how it works.