Handed over to Nimble after account was settled 2years ago

I have been battling since 18 Sept 2020 to get a paid up letter from MTN. I have send proof of payment numerous times, I have phoned, emailed to no avail. The next thing I know Nimble credit advises me that my MTN account was handed over. I have been struggling to resolve this matter for 2years with no answers and pure unprofessional service. Because of MTN's Incompetence I now have a bad credit score.