Ridiculous billing

I joined rain in 2020 pay as you use which they where very clear it was not a binding contract. I had connection problems the entire time and stopped using their services. I was on a pay as you use R479 option now they claim I owe them R2100 which they cannot send me a formal statement that shows how did they allocate such a ridiculous amount to my account. I would never advice any one to join rain as they will regret