First Abu Dhabi Bank [FAB] review: UAE GEMS Titanium card

I took a GEMS Titanium card which was advertised as 0% interest for 12 months on GEMS portal, to pay my son's school fees. After this, I started repaying the amount in equal monthly instalments. I started noticing hundreds of dirhams charged as interest to the 0% interest card. I have called Customer Care multiple times, they first said I needed to have called the bank to convert it to an Instalment plan. I have told them ok, I missed it I was not aware. But I have not defaulted on any payments, and I have paid exactly the same EMI if I had called the bank for instalment plan. Based on a formality, they have trapped a simple honest customer, and now my outstanding dues stand at thousands of dirhams for a 0% interest card! I have even paid the full amount now well within 12 months, wanting to close this card. But despite multiple complaints there is no resolution. This is an unfair practice to trap a customer and penalize despite no defaulting.

Why should only the customer be bullied by the bank on account of maintaining good credit history? Shouldn't the bank also have some official scoring based on unfair/negative practices, complaint resolution and customer feedback?