Issue with Boarding on Turkish Airlines Flight

Hello, on July 3rd, I went to the airport at 10:10 AM for the TK2843 Sinop-Istanbul flight scheduled at 10:55 AM. Before that, on July 1st at 11:17 PM, I selected my seat by paying an extra ₺45. On July 2nd at 11:09 PM, I received an email with my boarding pass. It was stated that I needed to have either a printed or mobile boarding pass to board the flight. However, due to being on vacation and sometimes being in areas with no phone reception, I did not create a mobile boarding pass, thinking I could get it from the kiosks or counters at the airport.

When I went to drop off my luggage at the airport, the staff at the counter said only those who had checked in could board the flight. The emails I received included my seat number and boarding pass, but I was not allowed to board the flight because I hadn't checked in. I was neither given a refund nor an option to change my ticket, and I couldn't board the flight because of them. I don't understand how I can be considered not checked in when I paid extra for check-in and received a boarding pass image in the email.

THY may have a policy of overselling tickets, which is fine, but my grievance was not resolved. My ticket was neither changed to a new flight nor refunded. I had to buy a new ticket by paying again.