Bad service

I have upgraded in August 2021 on the *** number, because of bad service I found myself with an migration fee of R 8000, of which I made agreements to pay it off within 12 months, I did exactly that, I submitted proof of payments every month until I was done paying. I requested a paid up letter nothing was done, what is most upsetting is that now MTN says I owe them 12 000, I must really say im beyond disappointed with MTN. I need help this has gone on for a long time now.