The ATT person is high. If you're opening a port (AKA port-forwarding) you are allowing the public Internet to access it. That means they have to be able to "see" it. If your port-forwarding is set up properly then anyone on the external interface (i.e. the "public" Internet) should be able to port scan you and see the open port. The fact that you say you've got port-forwarding set up but that your Sling isn't working and you can't see the open port is the problem. Unless the public Internet can "see" port 5001 open then viewing Sling remotely won't work. Did you set up the router manually for port-forwarding or did you let the SlingBox do it? How did you set the Sling's network properties?

I'm a bit confused w/respect to what you're telling us regarding canyouseeme. First you say that canyouseeme is indicating you've got port 80 open (why?). Then you say it's blocked. Can you clarify that?

Do you have keep-alives set up on the Netgear so that the PPPoE connection stays established? It won't matter if you've already got the connection up from your PCs...but it will once you get things working and you're trying to access the Sling remotely and the PPPoE connection times out.