Device Enrolment - Apple Business Manager

Hi there,

I have recently set up an Apple Business Manager account to manage all of our Company Phones.

The devices we have were not bought from Apple or an approved re-seller but were instead gifted over the years/hand me downs.

When logging into our Apple Business Manager account and selecting the 'device' tab along the side, I only have the option to enrol devices using a 'Apple customer number' or 'reseller number', which are not appropriate in my case.

I note there used to be an option to enrol devices using the serial number but that this is no longer available.

I have tried using Apple Configurator 2 using the 'prepare' steps but after a short while of 'preparing' the system states the following:

Provisional Enrolment failed.

[MCCloudConfigErrorDomain - 0x80EF (33007)]

And I cannot proceed any further.

Can anyone advise how I can enrol my devices onto my Apple Business Manager account?

Any help greatly appreciated - thank you!