I've precisely the same problem, starting some 5 months later than you. How/did you get this resolved? I've been chasing Xfinity for 5 weeks to get all the bits & pieces activated, but Flex is the last 1 & they're really clueless. I've spent too much time on this already, & this is my last ditch effort before I say forget it.
Greetings, @TPS0! I appreciate you taking the time to reach out to us here on the forums. I hope you are having an amazing day! I apologize to hear of the issues you are running into with trying to get the flexbox active. I would like to have a look into this and see if we can get this up and running for you. Can you please send me a PM with your first and last name so I may further assist you?
To send a private message, please click my name "ComcastMichael" then select "Send a Message" on the right side.