How to file a formal complaint against Assurant Xfinity Mobile Customer Service?

I’m currently locked out of my iPhone and have been for several days now due to negligence, incompetence, poor miscommunication, and very poor customer service from Assurant customer care team specialists. I initially contacted Assurant over a week ago to explain I am locked out of my iPhone, and I requested a receipt so I can provide to Apple to regain access to my phone. I called several times to follow up on my request, however, I still can’t get assistance. How do I file a formal corporate complaint on Assurant?