Interest being charged after account paid in full

I have been emailing and phoning since October without any success. I am an excellent payer with multiple accounts which I have been trying to paid-off accounts earlier than the original contract. When I received my last owed Amount Statement, I paid the amount in full on 12 Oct. November I received a statement with Interest charged after the (full statement) payment. Another in December. I contacted ConnectFin “we will investigate” is what I was told, with no further feedback. Both in November again in Dec. At the end of Dec I, decided that I do not want to carry the debt over in 2022. I decided to pay the FULL statement amount, again (with immediate payment) thus it will reflect on the bank statement the same date as payment (31 December 2021). When a company send a customer a statement of accounts, that amount on the statement is by law the owing amount between the two parties until the following month. I get a call 04 Jan saying my account is R400 odd Rand in Arrears. I explained the situation again and she asked me to please email her the proof (to the same email I sent the proof of payment to in Oct/Nov/ Dec). I have been emailing 4 email’s with no response. They keep charging me interest on a R0.00 balance account, this cannot continue, because they process the payment days later than the actual payment. They are steeling my money *******ly. I cannot seem to get a direct number or email address for a supervisor to sort this out, from any of the staff. They simply say they cannot assist me. They claim the statement amount is not the amount I owe (legally, that is actually exactly what I owe as per agreement, placed on paper from supplier to customer). They claim I have to phone for a settlement amount, how can this amount differ if the complete agreed contract has been paid in full. Plus extra interest I paid in December. This Is absurd.