I don't personally have any cable, but I believe you (Comcast customers) use the Xfinity Stream Beta app and basically use the Roku device to login to your cable subscription (thereby ditching the rented cable box). Other Roku users abandon cable altogether and use cable-alternative streaming services with their Roku like Hulu or Philo or Sling.
Sorry couldn't be of more help. Here is link for Xfinity Stream/Comcast info... https://www.xfinity.com/support/articles/activate-xfinity-tv-app-on-roku
Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
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