How to get someone to review my billing and transaction history with me

I have been in direct message with an Xfinity rep for 3 hours. I was not given the proper refund when my service was cancelled. They say it was due to a previous balance that was carried over from another month. When I look at my transaction history, it shows payments made every month going back to the start of the transaction history in October 2023. When I ask for specifics on when the payment was made, they continue to say the same thing "it is carrying over from a previous month". How can I get someone to review this with me? On my end, I see no missed payment. They will not give me more information on when the payment was missed for me to understand it. I am quite disappointed in the service provided my Xfinity. They bill a month in advance for your service and take a payment out even after the service has been cancelled. There is no way to remove an auto-payment card from your account unless you add a new one. I was charged for an extra month and now they are keeping the money and saying it was due to a previous missed payment without being able to pinpoint when it happened. If someone at Xfinity can help, please respond in this forum.