Lost phone “how to put on ‘lost mode’??

My gf iphone X been stolen on September at napoli-italy. I’d reported on “three store” by online! And also to apple store in regent street!. I gave them the IMEI no: And they all said they “blocked” the device!.

now when i apply for insurance for the iphone x, they said phone is on “lost” mode!

they want me to call apple to set-up to “lost mode” the stolen iphone, ? Can u please help me set-up stolen iphone x to “lost mode”? Thank u