Do you have a router installed? If so, have you forwarded those ports?
If not, then go to the
GRC ShieldsUP! site. When you get to the test page, type, "5100, 5678" in the blank box between the two rows of buttons, and click on the button labeled, "User Specified Custom Port Probe". When the test is complete, click on, "Text Summary", copy, and paste the result here.
If GRC confirms the ports are blocked you should:
• Check your firewall logs, and see if there are any probes to those ports from any IP address between 4.79.142.192 and 4.79.142.207. One of those is the source of the GRC ShieldsUP! probes.
Beyond that, I have no idea how to proceed: I don't have a Slingbox. I am reasonably sure that Comcast is not blocking ports other than a few low ports used for malicious activity; but I can't test that for myself.