Paid up letter

I paid my Wonga loan on Oct 2021 . Then Wonga sold my account to Nimble Credit Solutions. When I tell Nimble that my account has been paid in full, they want a paid up letter. When I request for the paid up letter from Wonga, they refer me to their portal, which is not updated. I'm furious and frustrated with this situation as it also affects my credit rating. I can't speak to a person at Wonga, their phone service is automate...I've requested this since 19 April till today