I need answers

On 29 August 2022 I asked Fnb to pay the balance of my credit card over a fixed term of 60 month's due to covid which impacted my income and the agreement was approved. On 25 March I saw that the interest calculated on a lower balance is more than what they calculated on the higher balance of the previous month. I discussed the matter with Fezile Mazibuko and Sibongile and their response is that it is the way they calculate the interest. It is the first time in my life that I hear you pay more interest on a lower balance. It is impossible to speak to a person and everything must be done via messages. The service I received was appalling and I will not recommend FNB to any person. If that is how they calculate interest then it is totally wrong and they are getting away with it.