Apple Business Manager
Change MDM server default device assignment in Apple Business Manager
An MDM server can also be set for specific device types. Devices that are already enrolled must be manually reassigned. For example, you can have a default MDM server for Mac computers and another default MDM server for iPad devices and another one for iPhone devices.
Change MDM server default device assignment
In Apple Business Manager
, tap Settings at the bottom of the sidebar, select your MDM server 
under the MDM Server list, then tap Edit.Select the device type to automatically add to the selected MDM server, then tap Apply.