FWIW, they don't "contract" / guarantee WiFi speeds. No ISP does. There are too many variables. Only hardwired ethernet connections.
Try calling 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.
Good luck !