Being asked to pay 2 installments in 1 month

Case number ***: I emailed FNB Card on 25/06 asking them to cancel my debit order (which i was forced to take) and told them i will be paying via EFT as i dont have a stable date for my other income, which results in debit orders bouncing from my current account. i was then advised by the agent that they debit order is compulsory (I cannot choose a payment method that works for me) and that this is on the T&Cs (which were not discussed with me when I took up this card). I told the agent to then change my debit date to the last day of the month, I said this on the 25th June. My debit order was not changed up until I followed up on 2/07 (before my billing date), FNB only confirmed on 10/07 and I was advised that I will have to make 2 payments because my statement has closed, so I have to pay on 25/07 and on 31/07. How come? If I requested this before the billing date? This is messing up with my credibility. this is not my fault at all as my instruction was not actioned immediately... Please sort out asap. please