Extremely Unhappy with FNB Bank

My FNB business account has been overdrawn. On the 4th of April 2019, I received a letter dated same day advising me that I needed to pay the overdrawn amount within 20 business working days otherwise this will result in them closing my account. I calculated that to 02 May 2019. I tried to make payments to reduce the amount owed. The system didn’t allow me to transfer the money to this account. Today, 11 April 2019, I called to check why I can’t pay to the account, I was told that the account has been closed. What happened to the 20 working days stipulated in the letter? I knew I had 20 days to pay it up and that’s what I was doing. I need FNB to honour what is written on the letter sent to me on the 04 April 2019. I need them to reactivate my account. I am very unhappy with this. The lady on the phone told me that there is nothing she can do until I have to tell or beg her to listen to my side of the story and follow up on the letter sent to me and it’s validity. This is really bad not to honour what you send to the client. It shows that you can just treat clients as you desire and any how.