The best thing to do if your iPhone is lost or stolen and cannot be retrieved using the Find My app is to erase the data to protect your sensitive information. If you have tried all recommendations to recover your missing device, then you can erase the device. Make sure to only do this once you have tried everything on your end as it can’t be undone. You shouldn't erase your device if it is covered by AppleCare+ with Theft and Loss.  To erase a device, just open the Find My app using a different device with your Apple credentials. Go to the Devices tab and select the device that you want to remotely erase. Scroll down and choose Erase This Device. Select Erase This [device] to confirm.