The best way to factory reset a DEP device is to do the following in the Device Console:

Settings > System > enable Re-enroll Apple devices in 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.

Once the device factory resets, follow the prompts and the device will be enrolled back into the Device Console.

How to factory reset an iPad that is enrolled in the Securly MDM via a DEP