I cancelled the contract but Im still Invoiced

I went to cancel my contract late october 2024 at MTN Evaton. I was told to return router and all accesories which I did. I received an invoice and I went to enquire.the consultant said its the last payment. I paid it in December. I asked her if I am done she said yes. In January this year I received another Invoice. I asked customer service and I have been sent to pillar to post, even now in Feb I still received and invoice. What is so hard with cancelling 1 contract. Why am I being invoiced for a service i have not used since November.