Alright, usually when a DEP device looses the management the device has to be wiped and set up as new or you can restore from a backup from the same device that had the profile on it before it was removed.
I tested your senario, remove the device from the dashboard, go to the DEP section in Meraki and the device was still listed, but it had "Meraki Management Profile Removed". I went to the iPad and went to m.meraki.com > entered network ID > entered my AD credentials to enroll > Downloaded & Installed the Profile. This worked for me! The new profile installed freshly over the DEP Profile (and it is still locked). It showed backup in dashboard and restored communication with the device. I know you said you have done this, but give it another shot. You can also try the Meraki MDM app if this method does not work.
Hope that helps!
Jared
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