activation lock Iphone 6s

I purchased an Iphone 6s that had already been erased and iOS 12.2 already installed. the Icloud find my Iphone had not been deactivated. I don't know original owner and the seller is of no help. All I get is phone has been disabled for security reasons warning on the activation lock screen. Is there anything I can do to remedy this issue or am I out the money already invested. I've tried every possible solution I could find online and on youtube. Can anyone help me please. I'm at my wits end. Thank you.