Formal complaint against the loyalty team

I spoke with a representative of the loyalty team on 10/01/23 and told them I was cancelling one businesses account and opening another. I was not told about a disconnection request form at this time. It was very clear what I needed, but I did not know to ask about the disconnection request form. I was assured everything was taken care of because I was opening the new account. I continued to be billed for the old account, thought it was fixed when I spoke again with someone from billing (autopay not set up for new account), no discussion of a disconnection request form at that time but I was told I am eligible for a refund and it would be processed, yet I still received another bill. As of today, I was told there is no refund being processed and I finally learned about the disconnection request form. Today I was told by the loyalty team I made no such indication of disconnect on 10/01 and will be charged through today 11/21/23, and somehow this is my fault?! Unbelievable. I am furious and have no recourse, and I have to pay for 7 weeks of service that I didn't want. This should not happen to other customers. How do I file a formal complaint?