Turkish Airlines Ticket Cancellation Due to Unreasonable Justification

I purchased a non-refundable and non-changeable round-trip ticket from THY for travel between Istanbul Sabiha Gokcen and Berlin. My travel plans changed, so I bought a separate outbound flight. When it was time for my return flight, I couldn't find my return ticket. When I called customer service, they told me that my return ticket was canceled because I didn't take the outbound flight. They even gave a nonsensical reason, saying you can't return from a place you didn't go. They told me to fill out a feedback form and wait for a solution. There are about 12 hours left until the flight. I made all my plans accordingly. Now I have to wait for their evaluation without sleeping. When purchasing the ticket, they offer separate prices for outbound and return flights. We select and pay for the ones that fit our travel plans. How is it logical to say you can't return from a place you didn't go? Should I wait for them to evaluate the situation, buy a new ticket, or what if there are no tickets left by the time I get a response?