Bad service at Standard Bank.

I bought a vehicle in 2018 through Standard Bank Vehicle finance on a variable interest rate. Since 2018 I think there were at least 3 -4 interest rate drops but my installment stay the same as well as my residual value. Std Bank don't know what to do and every time I complained I get a letter that said I cannot change the interest now. Must be done when deal is affected. Maybe they struggle to read. I don't want to change now - I want Std Bank to comply to the deal signed in 2018. What a frustration and bad service....