Billed for unused services

I have a rain simcard since June 2020, the sim contract is a month to month which is paid in advance every 1st of the month. I have requested for Rain to cancel their service as I am no longer able to pay for the services. I wrote to them on the 7th of February 2021 and I stopped using the sim. I then received an email that my issue was resolved dated the 9th of February 2021. I have been waiting for a bill dated the 1st of February to 7th of February when I stopped using the services. I have now been sent a bill of R804 stating my account is in arrears but I have not used their services. I am failing to understand how I get billed for services I haven't used on a month to month agreement. I have yet received any response from Rain except for an automated email response.