As the first step, we recommend you to add your Slingbox to your Sling Account. The following links will provide you with all the information you must know about this process.
Then, in order to fix the reported issue we encourage you to check and adjust some settings on your router. Just make sure to use both of the protocols (UDP and TCP) when creating or editing the rule for your Slingbox. Then, run the Setup Assistant for Internet Viewing.