It's been awhile so I completely forgot how to convert an iPad that was original not purchased under the Business Manager to a DEP iPad. I know you have to use the Apple Configurator.
Go to Solution.
Enrol using Apple Configurator then log into your ASM/ABM search for the device and assign it to your JAMF Server.
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Here is the link that I used to enroll my iPads manually.
Then you enroll them you will go to ASM or ABM to assign it to your MDM Server.