Angry client

Just now I wanted to make a payment to Institute of Internal Auditors (SA). I filled all the details on my banking app but was told to select public recipient option I did and paid the money worth R6 900. Now when i check the proof of payment, it has a different bank account. I called FNB and they kept on directing me to different pple and the last person cut my call. What should I Do FNB. Please sort this