Worst serve ever

I've been trying to resolve a issue with my FNB connect devices after I was contacted by Nimble collections saying that I owe them an outstanding balance. I then settled the outstanding balance the very next day on the 20th of May and requested a paid up letter. After calling the debt collectors I was told the account was handed back to FNB and that I should request a paid up letter from them directly, I then contacted FNB and after being sent from pillar to post I finally spoke to an agent that advised me to send a proof of payment and this would be forwarded to their legal department. After following up and trying to get the matter resolved for close to 2 months I still have not received a paid up letter, my credit report is also being affected negatively because FNB has updated the account as not paid for the last 2 months. I'm now being contacted by another debt collections company called Blake and associates saying that I still have the outstanding balance. This is after I had sent the proof of payment, aswel as the statement from Nimble collection showing that I have settled the amount. My credit report will now further suffer as they are telling me to send the proof of payment to them and they will start the process of updating the account. The worst part is that this will now stain my credit report and FNB will not do anything to rectify this issue