Unfair treatment from Trackernetics

In November 2017, I emailed Trackernetics to cancel my contract and to retrieve their unit from my car, as I was about to trade it in. They never responded to my request. But they continued to deduct the premium. I then reversed the premium as soon as they deducted and when they questioned I gave them the story that I emailed them to cancel the contract and retrieve the unit, but they said my contract only expired in April 2019. They have been deducting until now. I feel this is unfair to me as I am paying for the unit that I don't have.