FNB ruining my credit report

I had a credit card last year with FNB, I settled it and closed it, However, It still appears on my credit report stating that I still owe the FNB. I Experian to enquire about this, they notified me that I need to contact FNB to provide me with a settlement letter and that they must update my details on the credit bureau. I then contacted FNB, their call center agents kept transferring from one department to another, eventually, the agent attempted to generate the settlement letter, which I didn't receive, when checking my profile he said that it takes up to 3 months for changes to appear on the Credit bureau and also acknowledged that at the time of the call (November 2021) three months had passed for changes should be reflecting. Now almost a year this still appears on my credit report and their agents are quick to call us to upsell products to us.