Service from Hell

Services from Hell, I had a car tracker with Matrix Vehicle Tracking for 5 years. I asked them last year January to cancel the contract but they ignored me and still send me the invoice of which I didn't pay. In March 2025 I paid the settlement that they requested and after that they handed my account over to Nimble group and they have been harassing me ever since although I show them the proof of payment. Nimble group is asking me to provide them with the paid-up letter and Matrix Vehicle Tracking is referring me back to Nimble group. This is affecting my credit score on credit bureau.