How do I turn on find my phone
How do I turn on the find my phone app on a device that I am logged into but it is off on?
iPhone XR, iOS 13
Question marked as
Did you enable it BEFORE it got lost? If not, then you will not be able to do so remotely ... and if that's the case, your only option is to go look for it.
Ref: If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support