Xfinity Mobile Support

How can I get a hold of a manager for Xfinity Mobile?  I've been promised that Xfinity would honor a past promotion which has been documented in their system and now after 2 weeks of talking to multiple "customer service" agents, they are not willing to honor it.  How can I launch a formal complaint so an actual person that cares about customer experience will help and look into this for me?  So far, this has been a terrible experience.