Click on this link for general information about what to do "If your iPhone, iPad, or iPod touch is lost or stolen" --> https://support.apple.com/HT201472
If Find My iPhone is offline or not working' click on the link to 'Get help when you don't see your device listed, or you see an unexpected device or an alert that says your device is offline.' --> https://support.apple.com/HT204233
"Use the Find My app to locate a missing device or item" - https://support.apple.com/HT210515
- "Don't see your device? You might not have Find My [device] turned on. If your device is powered off, the battery has run out, or more than 24 hours has passed since its last location was sent to Apple, you also can't see your device's location."