Its on their approved devices list so you might want to call and talk to another rep. And I know quite a few people who have one on xfinity. 

I posted a link to their approved devices list (which its on) but for some reason it got flagged for moderation. You can google it. but the s34 is approved, and I know quite a few people who have it on xfinity. Might try calling in and talking to a different rep versus one who doesn’t even know whats approved. 

If you are trying to activate the modem I would recommend try to do it yourself through the Xfinity app. Please ensure the S34 light is solid blue before starting the activation process.

For your convenience, you can check the list of approved and compatible services under Xfinity through this link.

http://arris.force.com/consumers/articles/General_FAQs/Service-Provider-Modem-Compatibility-List

For further support, you can visit us at www.arris.com/consumersupport