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Trusted device on the iPhone shows list of devices where you have logged in with your Apple ID. But recently many iPhone/ iPad users are facing that iPhone 12 is not showing Trusted device list. The issue seems to be widespread as different devices like iPhone 13,iPhone XR, iPad pro are also reported with same issue.
In the setting app iPhone users is seeing the message: “Trusted devices list is not available”. The issue started intermittently and the there are no devices list on the Trusted device section on the iPhone 13 and iPhone 12 Pro Max and also the issue is seen on the iPad Pro.
The missing trusted device issue seems to be older issue and the same issue was reported with older version of iOS 10 and iOS 13 version. Apple should take care of this issue and fix them permanently.
One common solution to fix trusted device list unviable is to restore you apple id settings. The issue could be linked to Apple ID issue and related to your account issues. To fix that try logging out of your Apple ID.
It seem that the trusted devices are missing from the iPhone on the Home Networks. Please try to switch to some different WiFi Network or turn OFF WiFi on the iPhone & switch to mobile data and the list reappears.
If the issue started after recent update then changes which are made to your device, sometimes items(i.e. connectivity) become manipulated, technically. So, update your iPhone to new version of iOS 15 etc.
If you are still not able to see trusted device, please contact apple support and explain your issue to them.