Impossible to cancel ********** upgrade and account keeps on accumulating

Cancellation impossible. They signed me up for an "upgrade" via call Centre, 10 days later I realized I'm expected to buy a modem for home internet at a much higher cost too - not remotely the same package I had. I phoned, emailed to cancel within cooling off period just to get zero response. I've cancelled the debit order with my bank after they managed to debit for the sign-up fees (never communicated either) Every month when I receive this accumulating invoice/statement I query again with absolutely NO response. I refuse to pay for your lack of customer service to cancel the product. I've never used 1kb of data yet you're expecting me to pay before you cancel although you didn't respond within cooling off period?? It's outrageous! Have anyone ever had success with cancelling this ****? Do I need to get the services of a lawyer? I'm at my wits end...