***** company works outside the law

Good day

I have a huge problem with this company. In the year of 2017 I was unemployed. I found a job and have made 3 payment to the company due to the back log. Only to be informed that they add interest on the 24th of every month. I make a deposit of R300 as that is all that I can afford. For 3 months only to find out that my R900 deposit I have made has not effected my account as my account is still over R4100. In what country do you add your interest before every month and getting sms threats and phoned due to people not doing there jobs. In any law the law states that you can add interest when the account exceeds 30 days not before the time.