Unsettlable account

I took out a car insurance in 2014 and I changed the car in 2018. The insurance was contracted to fit a car tracker by Altech. They notified me that the tracker is not working in my car and the technician would come and assess, nobody came until I sold the car. Then NBA started calling that I owe Altech, I ended up making an arrangement with them to pay the money. On the 27 October 2021 I called them to make an arrangement but I was referred to the Nimble app that gave me the settlement amount. I then paid the amount and an agent called me to say that I will receive a paid up letter. I received nothing until today when I received a call to say that my account is overdue by 220. I asked what is for and they said it's interests. I asked interests for what because the account has been settled though the tracker was not working for years