Contract cancellation

I cancelled my month to month contract at MTN Melrose Arc 29/07/2024. I was told that it has been cancelled as I was upgrading to another router with higher speed. They told me that I will be billed for two router in August because I cancelled the other one late ( I was fine with that). To my surprise, in September I got billed for two routers again, when I called they told me that I never cancelled. I asked them to cancel and refund, they told me that there is nothing they can, I have to go back to the store. Pathetic service and such an inconvenience as I will have to spend my money to travel to the store.