*****ing people IN ADVANCE

I was billed in advance by rain. I had to make a payment on 30/03/2021 before I received my simcard. I received my SIM on 31/03/2021 and it took 24 hours for it to be connected, therefore 01/04/2021 was my first day of connection. Now they send me an email saying they bill in arears! How??? If their feedback is correct, what was the initial payment I paid for for?

RAIN Please return my money immediately and remove my billing details from your system.