Since you are able to get the Slingbox up and running when connected to the first router, we recommend you to check the settings on the second router.
If the Slingbox works fine on the first router, you will need to adjust some settings on the second router. We also recommend you to check the upload / download speed when trying to get the connection on the second router. Keep in mind that once you get the connection through the second router, the upload / download will be affected.
Finally, we recommend you to connect the Slingbox in a different network and test the connection. This test will allow you to know if the issue is related to your network settings.