MTN *********** customers by double charging when contract upgraded

I upgraded my old contract for my cell phone sim with MTN on 05/11/2021. The old contract had expired months prior to this date and I was on a month to month basis with them for services. They commenced my new contract on 05/11/2021 and my bill dated 27/11/2021 showed charges from 05/11/2021 to 26/12/2021. I was fine with this billing as MTN charges in advance for the month so they charged pro rata for November and a full month for December 2021. My problem is their charges for the last month on the old contract. Their invoice dated 27/10/2021 shows charges up to 26/11/2021 based on their advanced billing method. The debit order went off for the full amount as per their invoice. I expected a refund from 05/11/2021 to 26/11/2021 on the old contract as it was replaced by the new contract on 05/11/2021 and therefore ceased to exist when the new contract started. From the beginning of this month I have been trying to get some sense from MTN on this matter as they are adamant that their is no credit due to me. They fail to see the double charges for period 05/11/2021 to 26/11/2021 and feel that they are entitled to charge a customer on an old contract for period a upgraded contract is running. This is a very ********* way of doing business and they are *********** customers by doing this. I cant be the only customer they are *********** in this manner. I repeatedly asked the customer services agents to refer the matter to a senior accountant at MTN to look into matter but they have failed to do this.