Turkish Airlines Didn't Refund the Extra Seat Fee

A few days ago, I purchased a ticket for my flight from America to Boston through their mobile app and paid an extra ₺4,000 for a seat with extra legroom. Later, they changed the flight time, which I accepted. However, they did not assign the extra legroom seat I selected to my new flight. Currently, I don't have a seat, and when I try to select a new one, they are asking for an additional fee. Every time I call customer service, I get different responses, but my issue is never resolved. Either refund my seat fee or assign the seat I purchased to my flight. If my issue isn't resolved soon, I will file complaints with the necessary authorities. They took my money but neither refunded it nor provided the seat I bought. This is unacceptable. A supposedly large company like THY should not have such poor service. Please resolve my issue urgently.