Rain 5G subscription

I have used Rain 5g for almost a year, I later cancelled the service as its a month to month, even made arrangements to collect the router, but now the say I owe them R3300 for subscription on months I didn't even use the services after cancelling. I keep getting calls for collection and I ask them how did the bill get to R3300 as it is a month to month deal...if don't they don't connect me , so how did they allow it to run for 4 months after cancellation ?