The main issue in this case is that all the Linksys routers have an IP Pool Range that goes from .101 to .149. If you are using the .254 (Slingbox default IP address), it will not work for Internet Viewing, since it is out of the range.
You just need to make sure to use the right IP address provided by the router for the Slingbox, in order to create the rule. This way, you will be able to set up your Slingbox for Internet Viewing.
The following third-party link will provide you with the steps to follow.