Unfortunately this is the only article that we have about setting up Internet Viewing when you have multiple routers,
First, you need start the Setup Assistantand during the manual setup for Internet Viewing you will get the default gateway, probably 192.168.X.X, so you need to open the an Internet browser and paste that IP on the address bar, that will give you the access of the Linksys router, follow this steps to setup Internet Viewingon the Linksys. After that, access the "Status" tab on the router and copy that default gateway and open a new window, paste the IP to access the Modem and follow this steps. Remember that you need to create the rule on the modem for the router, not for the Slingbox; you need to use the same port in both setups (5001).
Then, just run the Setup Assistant with the IP that you used for the Slingbox during the Linksys Internet Viewing setup and that will do it!
Let us know how it works.