Rain has debited more than what my monthly payment agreement is with them, they overcharged me and then come back to say I owe them, says I am in arrears

Rain has debited more than what my monthly payment agreement is with them, they overcharged me and then come back to say I owe them, says I am in arrears. They just debit R800 + from my, I don't have internet connection, when it comes back is interrupted, they app is not working, you can't log in is always on error. What is exactly happening I don't know. Is very frustrating