ADT claiming I owe money after cancellation. Handed over to Nimble Collection Co.

I cancelled my ADT contract on 11 November 2019. Apparently there is an outstanding balance owing because ADT only confirmed my cancellation on 17 January 2020, more than 2 moths later. As far as I am concerned, my account was up to date. Why should I pay for your delayed reaction and poor service? And then, with no communication coming from ADT regarding this, not a phone call or an email, a whole year+ later, I am handed over to a collection company Nimble. ADT, I'd like response, please explain why I owe you money, I really DO NOT feel I owe anything.