Failure to comply with debit dates!

I have an FNB connect sim and although I have contacted them numerous times, they continue to debit my account on any date they feel like. I am a pensioner and receive my funds on the last dy of the month, which is why I set my debit orders for the 1st of the month, but FNB connect will debit my account any date from the 20th of the month. Who do they think they are? I am so sick of this. Have never had this experience with anyone else. A debit order is set for a certain date for a reason and surely they should respect their clients instructions!!! Really not happy.