It's a beta, and beta's aren't guaranteed to work. You tell the maker there's a bug in it, and - with some luck - they will fix that in the next release. Did you file that bug report?
I assume all conditions mentioned in https://www.xfinity.com/support/articles/xfinity-tv-app-on-roku-faqs are fulfilled. If not, it makes sense that it doesn't work.
I purchased a Roku Premium Streaming device after reading about the availability of a Xfinity Beta App I could use to access the same programing I get through my Comcast subscription without needing a cable box, and most importantly having to run coax through an inaccessible exterior wall. After two months of trying and countless of support exchanges with Comcast & Roku, I have been unable to make this work. The available literature in Comcast's own website states that this set up will work with customer owned modem and wireless access points. It seems that the app expects to connect to an Xfinity in home wireless network. I have Xfinity internet, but not rented modem or wifi routers. Everyone I have spoken with at Comcast just says it should be working, but so far I cannot get past the activation step. I've had Comcast techs out to no avail. Has anyone run into this?

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