Still Billing me after Account was cancelled and device returned

Rain is the worst internet service provider in South Africa. I thought that no one could beat Telkom but I was wrong. Last year June I app**** for a Rain Month to Month contract. The 5g did not work and I requested cancellation. The device was collected but Rain then started to deduct funds again off my account. All the Call Centre agents can say is that Billing is investigated and I am unable to contact Billing myself. I want a full refund of all the deductions you have made.