Turkish Airlines Rescheduled My Flight Without My Knowledge and Provided Lack of Support

I purchased round-trip tickets from Istanbul to Mardin for myself and my spouse through THY. I got a very good deal because of a promotion. All my emails, SMS, tickets, and even the check-in tag on the day of the flight indicated that the departure time was 15:20. However, upon arriving at the airport, I found out that the flight had already taken off at 15:05. Despite being a victim of an early departure, I had lengthy discussions with THY. Their response time to a single feedback is already one week, and I received no support after 6-7 feedback attempts. They are very unresponsive and indifferent. THY only offered me a one-way voucher for one person. I told them that my vacation was ruined because of them and asked them to cover my return tickets as well, but they claimed they did not see any issue caused by THY. Who else changes the flight time or decides not to board a passenger who has already checked in?