Try callingl 1-800-comcast and ask a rep to completely remove the modem from your account and to "re-provision" it from scratch. Also ask them to confirm that the provisioning and account data in their database is correct and complete and that the correct configuration / bootfile for your subscribed to speed tier is being pushed to the modem.
Thank you for posting to the Xfinity Forums. Sorry for the experience you're having. I can help by removing the old modem and reprovisioning the new modem to make sure yo're receviing the speeds you're paying for. Can you please send a private message including the full name as it appears on the account and your full name if different? To send a Private Message, please click "Comcast_Support” and click “send a message".