In your Meraki dashboard if you go to Systems Manager > Manage > ADE can you see the device if you search for it using its serial number? If you can see if you should be able to simply Assign Settings using the buttons at the top of that table on the page.
If you don't see the device in this list either its not loaded into ABM or you haven't assigned said device to Meraki Systems manager MDM.
DM me if you like I have a lot of experience with Apple devices and MDM's.
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