Thanks for posting in the forums! I'm sorry to hear you are unable to change channels via on-screen remote. I'm under the assumption that nothing has been changed and the remote suddenly stopped working.
Just to be sure, does the on-screen remote look the same? Also, do you have the IR Emitter cable connected from the IR port on your Slingbox and sitting in the front of the face of your A/V source?
Below is a link to an article that explains how to test if the IR emitters are working properly. The article explains how to do it with IR blasters for a Slingbox SOLO or PRO-HD, but it is the same test for the 500:
If the results of the test show the IR Emitter is working, could you reply back telling me who your cable provider is?
Hope this helps!
The Sling Moderation Team