If you're running into activation issues with Google Fi on your Xiaomi 18 series phone, the problem may be related to the version of the Google Fi app you're using.
The solution is to install an older version of the Google Fi app instead of the latest release.
The Fix
The solution is to install an older version of the Google Fi app instead of the latest release.- Uninstall the current Google Fi app or uninstall its updates, if applicable.
- Install the older Google Fi app version provided in the XDA link below.
- Open the app and try activating your Google Fi service again.
- It may ask you to update to the latest version, do so and activation should be achieved.