Why do I have to back pay

First of all the network is bad Second thing is I don’t remember signing a contract or agreeing to a term that said if I didn’t pay the wife the previous month I will have to pay it the following month .why am I paying for services I didn’t use ?the wifi was not working for the whole month so what am I paying for ? We must pay for a service we didn’t use thats crazy I was gonna understand if it was said that even when u haven’t paid the wifi still works then you guys have the right to double charge because I used the service without payment . R650 is a lot of money for me to pay something I didn’t even use aiii no

Im disappointed