Find My IPhone
Why doesn’t iPhone build something in the actual phone for you to be able to find your phone. Someone steals your phone and removed the SIM card, you can’t “find your iPhone!
iPhone 12 Pro Max, iOS 14
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I don't think a SIM card has anything to do with the feature. I can find mine and there's no cell service on it. It's just then limited to what it can find in terems of known wifi.
Click on this link to read how to "Use the Find My app to locate a missing device or item" --> https://support.apple.com/HT210515
The chances of locating a device depend upon which features you selected in:
Set up Find My on your iPhone, Mac, and other devices - https://support.apple.com/HT210400#enablefindmy