Thanks for posting in the forums! I'm sorry to hear you are having troubles with your program guide.
The reason you are experiencing this issue is because the the A/V Source you are using is not supported. The remote works because you have a custom remote but the program guide is not made to be compatible with the set-top box you are using. You may experience this issue until it is added to our remote control setup.
I would recommend submitting a remote controls request:
Have a great day!
The Sling Moderation Team