I was trying to get a Roku working with my xfinity service. The app is in beta.
The list of supported devices is in this article: https://www.xfinity.com/support/articles/activate-xfinity-tv-app-on-roku
It listed the Roku Ultra 4660 as a supported device. However, Roku no longer makes the 4660, it’s been replaced with the 4661. I was hesitant to try something off the supported list, even though the 4661 is basically the same hardware as the 4660 and they throw in a pair of headphones. The Ultra 4661 works just fine.
I ran into another issue – I’m in the middle of a move and I had 2 xfinity accounts linked to a single username, one at my old property and one at my new property. I only kept internet service, not TV service at the old property. The xfinity authorization link at xfinity.com/authorize isn’t smart enough to understand multiple accounts, unless both accounts have TV service. I had to have xfinity customer service unlink the accounts, then the Roku worked.