Unable to manually add device serial numbers in Apple Business Manager since iOS 14 released

We have a mixture of DEP and non-DEP devices split across two different MDM servers, as we are migrating to Intune. All the DEP devices have been migrated to Intune MDM. We now need to move the non-DEP devices to Intune, but cannot manually add the serial numbers into DEP, as the option is no longer available.

I can confirm that the new licences for IOS 14 (released on 16.09.2020) have been agreed to in our ABM, but not until 21.09.2020.

Would this time delay have removed the option to add device serial numbers , even though it has been agreed to?

How do I get 'Device Assignments' reactivated in my ABM to add device serial numbers manually?