Interest Calculation

I need to understand how do you calculate your interest, early 2018 I was handed over to by TFG with an open balance of R12997,90. I have thus far paid R8143,55. I agree I have missed a few months since then. However in wanting to continue to settle my account I am told my balance is R10523,55 + R585,35 to settle account, meaning in total I owe =R11108,90. Communications fees from 2018 to 2020 is R410,46. Now here is the interesting part, interest gained since I started paying in 2018 is R5259,27 + R4853,82 balance of TFG + R585,35 settle amount + R410,46 communication fees = R11108,90 I need to pay to settle my account. Outside the R8143,55 I had already paid.

Now I would like to know how is the R5259,27 interest gained calculated with just two years of holding my account, as this kept growing while was paying you month to month.

Your consulted who called me had different numbers from above which I took in the statement.