Old devices not bought through ABM can be inserted into Intune but time consuming.
- Setup Apple Configurator 2 on any Mac
- Connect it our Intune information (https://docs.microsoft.com/en-us/mem/intune/enrollment/apple-configurator-enroll-ios)
- Add the device serial to the profile on Intune & assign the profile (Apple Config profile) (Devices > Enroll devices > Apple enrollement > Apple Configurator > profiles)
- Deploy the profile to the device (one already setup) via the Apple Config app
- From ABM, find the device in the 'device' tab
- Edit device management
- Assign to Intune