The best way to factory reset a DEP device ist to do the following in Control Tower:
- Settings > System > enable Re-enroll Apple devices into the previous device group > Save
- Devices > click on the name of the device group > put a check in the box next to the iPads you wish to factory reset > Select the Erase Device button at the top of the list.
- enable Clear activation Lock
- enable Delete Devices from Control Tower
Once the device factory resets follow the prompts and the device will enroll back into Control Tower.