Help & troubleshooting for channels on your Roku device, including adding/removing channels, logging in to, authenticating, or activating a channel, channel-specific playback issues, assistance contacting channel publishers to report issues, and adjusting channel-specific settings.
I just tried it on my Ultra and it's working fine. I had to do a system restart when my Pluto app quit working maybe try that. Maybe try uninstalling and reinstalling. Another option is the STIRR app which has Comet channel on it.