Sooooo fed up with CellC

I had a contract with CellC which then fell into arrears due to financial strain. I then wanted to settle the account from last year July however I was struggling to get a settlement letter from them. I eventually got the letter this year in August and I paid up the account. CellC decided to hand me over to the Nimble Group after I settled the account. Nimble contacted me on October 14th 2021 regarding the CellC account, two months after I had already settled with CellC. I'm in the process of trying to obtain a home loan however due to the credit listing from Nimble, I am stuck. I am unable to get a paid up letter from Nimble even though I've sent proof of payment. Apparently it takes 7 to 14 days to allocate the funds and issue me with the letter. It is currently the 16th of November, a month later, and the letter is still on its way. While I've been losing possible homes, and not knowing which school my children are going to go to next year as I don't know where I'm going to live because someone refuses to issue me with a damn letter for an account I have settled. I just need the paid up letter to give to my bank to process my application. Is this something impossible to get?