Hi,
It depends what solution was want to implement and the requirements? Do you have onpremise users and groups that were sync across ? Are you looking for cloud only solution after the ransomware?
The approach will be to rebuild the AAD Connect server import the configuration if you have from previous setup and match the users via UPN or whatever attribute you set - how-to-connect-install-custom
ADDS Service is Cloud managed domain service with some limitations however it depends what your end goal is so design and plan it accordingly.
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