Turkish Airlines Full Flexible Ticket Refund Issue

I purchased a round-trip ticket from Turkish Airlines to New York. I did the same in September 2021. The outbound flight allows for a paid refund and change, while the return flight, which is fully flexible, promises a full refund. Date changes are also free because I might need to travel to two states, Miami and Texas, for work before returning to Turkey. When I arrived in New York and tried to cancel and refund through their website, it showed a refund of only ₺800. When I called customer service, they said they couldn't do anything and that I would have to pay $190 even for a date change. I paid an extra ₺18,000 for a fully flexible ticket for two people. If they had mentioned this on their website, I would have chosen the cheaper option. Now I am left in a very difficult situation.