Paid account not allocated

I made a payment towards my instalment on the 12th of December and keep getting sms that the account is in arrears, when I call RCS they in fact did receive the payment but told me they only work out instalments on the 12th of each month so they decided to allocate the payment to the outstanding balance, I was told I must pay after the 12th if I want to pay the instalment amount,<br> they cannot re-allocate to the correct instalment, my credit score has now been negatively affected even though I made payment way ahead of time.<br> Asked to speak to a manager but was told the manager is busy and will call back.<br> I want to close this account.