Thanks for using our answers forum! I'm sorry to hear that you're having an issue setting up internet viewing with your Slingbox Classic. Double router configurations can be tricky sometimes!
Here is an article from our support site which should assist you with this particular issue:
Hope this helps!
The Sling Moderation Team
On the slingbox you have connected to the second router, try re-running the slingbox's setup assistant wizard and change the port under the advanced internet viewing option.
After that, open the new port that you set on both the second and primary router. (Same as in the document you read.)
If you still run into issues, try changing the IP address of your secondary router to be on the same subnet as your primary (Not 192.168.2.x, but 192.168.1.x), or setting the router as a passthrough/bridge, and just open the port on the primary. Unless you have some specific reason for them to be separate...?