A few suggestions. First, you do not provide information about your MDM so direct guidance on the MDM side is not possible. Next, you also do not detail which version of Apple Configurator you used to side load the iPad. If you are using the Mac version, then you cannot pre-assign to a DEP token. If you are using the iOS version, then you can assign the device to a DEP token at time of side loading.
Here are a few things to check.
1: Log into ABM and search for the device record. When/if you find it, confirm that it is assigned to the Server token shared to your MDM. As noted above, if you used the Mac version of Apple Configurator, the unit may appear in ABM as assigned to Apple Configurator. This is incorrect. Verify that the device is assigned to your MDM.
2: Log into your MDM. Find the pre-stage policy (enrollment policy) and verify that the device is scoped to it. Remember, a device may be associated to the DEP token but that does not mean that it is scoped to your automated enrollment policy. A institutional device left out of scope of the automated enrollment policy will behave like a retail asset. While reviewing your pre-stage, also check to see if you are allowing automatic assignment of new devices to the pre-stage. This is a standard option across every MDM I've run across. Here is an example from Jamf Pro. The production pre-stage allows all new devices assigned from ABM to be scoped to the pre-stage.
Hope this help you find the omission in your setup. Again, if the device is visible in ABM and actively assigned to your DEP token, that is the first step. If the device is being reported as "released," then you must recapture it with ABM. If you allow "release on deletion from MDM," then during your testing and validation, you are forcing a release from ABM when you delete the device record from your MDM. I encourage all new environments to disable this (this is done in ABM).
