Hello @Kanika_Bhatia,
To completely remove a iOS device from management, the steps in the Uninstalling Systems Manager and Removing Managed Devices documentation for iOS/iPadOS should be performed. The steps required will vary depending on whether or not the device was ADE enrolled.
If the device is no longer seen as managed within the dashboard, you could follow the general steps to completely erase the device as seen in the Unenrolling from a MDM Provider - iOS documentation. If these steps are not working, it is recommended to reach out to Apple Support in order to discover the steps to put the device in the DFU or recovery mode in order to completely wipe it.
Best,
Feli
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