Turkish Airlines Check-In Cancellation for Flight TK7375

On October 29, 2023, we completed our check-in at 8:30 PM on October 28 for the flight TK7375 departing from Rize Airport at 6:50 PM to Sabiha Gokcen Airport. We had purchased our seats (2E - 2F) along with the flight tickets, completed the check-in process, downloaded our boarding passes to our phones, and arrived at the airport on time the next day for the flight. On the morning of the flight, we received an informational message stating that our seat numbers had been changed due to operational reasons, and our seats were changed to 1F and 2F. When we arrived at the airport, the system did not read our mobile boarding passes at the gate. The staff at the gate informed us that we had been placed on the standby list after searching for our names. Despite having completed the check-in process and pre-booking our seats, they hurriedly tried to find us seats on the plane, as there were no available seats left. My friend is afraid of flying, so we purchased front-row seats. However, our check-in was canceled, and we were placed among the standby passengers. They barely found us seats at the very back of the plane. The plane was completely full, and we couldn't even find space for our hand luggage. I called customer service, but they didn't listen to me and told me to file a complaint on your website. Even though we had completed the check-in process in advance, it seems we need to arrive at the airport 5 hours early to confirm at the counter...

Why do I have to deal with filing a complaint when it's not my fault? Why couldn't I travel with the seat I paid for in advance? What will happen to the money I paid for the seat? What about my inconvenience?

Passenger names:
Senem K*** S***
S**** S*****