After the iPad has enrolled MDM via DEP, you can mark it as Shared iPad in order to use Managed Apple ID. 

1. Check if the device has installed MDM/Remote Management via DEP
   
   Apple > DEP > DEP Device
   
   
2. Set as Shared iPad
   
   Action > Set Shared iPad
   
   
   
   
3. Wipe Device and re-enroll the device as Shared iPad
   
   Device > Device Management > Select the device with Shared iPad label > Advanced > Wipe Device