Marketing lie

when I signed up with Rain, the selling point was that it was a month to month business model. at times I did not have funds when the debit order went down, and as expected, my internet connection was stopped, and once I made a payment, I would be reconnected.

now I call them to cancel the account, as I no longer require the use of their service, they tell me I owe them for 2 - 3 month's worth of internet connection. now it is my problem that their system is not reliable in detecting when funds was not paid, and then stopping the service (as it previously worked)