Turkish Airlines and AnadoluJet Ticket Change Issue

I purchased a round-trip ticket from Turkish Airlines for a flight from Ercan Airport in Lefkosa to Sabiha Gokcen Airport on April 3, 2024, and a return on April 13, 2024. Later, I needed to postpone my departure ticket to April 4, so I logged into the THY app. However, I realized that the THY and AnadoluJet apps are separate. I then downloaded the AnadoluJet app to change my ticket, but for days, no flight information was available on either app. Today, I finally saw the flight tickets and tried to change my ticket, but the app persistently did not allow me to make the change. I decided to cancel my departure ticket and buy a new one, but despite having Prime Fly service, the ticket fee was not refunded to me. When I called customer service, I was told that since I bought a round-trip ticket, both tickets would need to be canceled, and the fee could not be refunded. I don't understand why I had to pay an extra ₺400 for a ticket I will never use; my intention was not to cancel the ticket in the first place. I resorted to this because I couldn't change my ticket in any way. The customer service representative told me to write a feedback form, but mentioned that sometimes the fee can be refunded, and sometimes it can't. I don't understand anything about this new system. I request that my grievance be resolved.