It seems that you are having some issue with your Slingbox PRO and the Internet Viewing feature.
As the first step, I recommend you to reset the Slingbox in order to get rid of all settings. The following link will provide you with the steps to follow.
Then, wait until the Slingbox shows solid lights and run the Setup Assistant for Internet Viewing. Just make sure to run the Manual Setup and use the TCP protocol when setting the rule on the router.