Disgusting Rain

I have cancelled my rain contract at the end of February 2021 this means my last debit was supposed to be at the end of March 2021 which was done and now at the end of April they debited me again for a service I never used. Upon attempting to reverse with my bank I was advised that Rain submits it as an online purchase and not a debit request so my bank cannot reverse the debit. When calling Rain to request a reversal I am told that I am debited at the end of April for March usage which is utter bull as I have been paying from the day I took up the deal how can a company bill you two months in arrears?