someone activated lost mode on my iphone with a password, how can I remove this password?

I guess my icloud id was hacked, because lost mode was activated on my iPhone with a passcode which i can not remove thrugh my icloud.

I have reseted the password on my apple id and removed the lost mode on the iPhone but it still requires the password set by hacker. I have no idea how to unlock the phone without reseting it :/

anyone any ideas?