Continued Charges After SIM Subscription Cancellation Request

I subscribed to a Hey SIM card plan in December of last year. At the beginning of January this year, I submitted a cancellation request through their website. I subsequently received a confirmation email stating that my subscription had been successfully cancelled.

However, despite this confirmation, the company has continued to charge me. Even after I revoked the direct debit authorization with my bank, the company has continued to send me invoices requesting payment.