Signing in to Apple TV via Comcast's 3rd party app feature

I have subscribed to Apple TV through Comcast's recent offer and am unable to log in to Apple TV via Comcast's 3rd party app menu. It presents me with log-in screen and as I initiate that via my mobile device (I already have an Apple ID) it "loops" by indicating on my iPhone that I'm logged in but returns to the sign-in screen on my TV. 

I spent an hour with Comcast support yesterday and have already done the following:

• Replaced my Comcast DVR box
• Rebooted all TV comcast boxes in the house
• Refreshed the system

The Comcast tech finally sent me to Apple. The Apple tech (another hour on the phone) walked through the login plus the following:

• Had me try signing in with a my husband's mobile device
• Confirmed that I can access Apple TV on Apple devices

They are now sending me back to Comcast for support on the assumption that the problem is in the sign in interface residing on Comcast's system. 

Has anyone else had and resolved this issue?