Trade in credit cannot apply

I traded in my old iPhones in September, shipped my old phones back for 1100$ credit for my two lines. Since I shipped my iPhones, I made over 10 phone calls to Xfinity agents, just to ask when the credit can be applied to my bill. All they told me is to ask me to wait and the credit will apply after a few months. After over 3 months, when I called again, they told me I CANNOT get credit because my plan somehow downgraded to unlimited for 2 days, before I called the agent to set my plan back to premium again. And they refuse to provide me the monthly credit anymore. 

As a platinum customer of Xfinity, I'm really disappointed with the trade-in process, and the agents are full of lies, and very few can solve the problem with professionalism. 

I just posted here to ask if anyone is capable enough to help me solve the problem.