It seems that you are having some issues in order to get the remote connection to your Slingbox Classic.
First of all, the upgrade to the Slingbox SOLO has nothing to do with your Slingbox Classic issue. Since the Slingbox SOLO has already been successfully configured for Internet Viewing, I recommend you to reset the Slingbox Classic and make sure to use a different port number when adjusting the router settings.
The following link will provide you with the steps to follow through the whole process:
Even though you are dealing with the Slingbox Classic, the instructions are the same to follow.
Let us know how it works for you.