Bad service

I took a loan with the above mentioned bank and I wanted to finish my installment before the agreed period I visited the branch to ask they explain to me that they call the payment adhoc, so if I pay the interest is going to drop but the monthly installment is not going to change, we agreed I went ahead and paid but to my surprise nothing has changed I visited the bank again no help I got, the statement changed they told me the interest would drop if I settle the account then I asked for settlement amount and the amount was more than the balance, I paid for 14 months but to their system is only 11 months