I cannot locate my iPhone with the app
How can I find my iPhone?
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Sorry to hear that you have lost your iPhone.
To locate it in the Find My [Device] app, you must have activated the Find My app BEFORE the iPhone X was lost. Here is some information on that app If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support.
Note that your lost iPhone must be able to make the connection to the internet via Wi-Fi or cellular connection so that it can mark its location and broadcast it.
Axel F.