Second hand used iPhone İcloud problem

Hello ,
I'm a good apple user. Last summer I bought a 2nd hand iPhone 6 plus 128GB model phone. I forgot to log out of the iCloud account and returned it to the factory settings. I can not reach the old owner of the iPhone. They said they could not help me when I contacted the company where the phone was purchased. I regularly update this phone. Maybe I can use the old owner account to remove it. But the old owner does not sign out and the phone stays on the Activation screen right now.
Please help me. If you want, I can send you the citizenship certificate of Turkish Republic Citizen.
Have a nice day.
IOS version 11
capacity 128gb
Color space gray
Model Iphone 6 plus
Imei: 354381060125255