Unable to un-hide users from GAL

curious7 used Ask the Experts™
We have user mailboxes in O365.
Some email users mailboxes are hidden from address list and I wan't to un-hide them
When I try to do this from O365 console then get the following error :-
"The operation on mailbox failed because it’s out of the current users’s write scope. The action ‘Set-Mailbox’, ‘HiddenFromAddressListsEnabled’, can’t be performed on the object because the object is being synchronized from your on-premises organization. This action should be performed on the object in your on-premises organization."

I can't modify the "msExchHideFromAddressLists" attribute for these user accounts as this attribute does not exist on the on-premises domain controllers.

How can this issue be resolved? Meaning how can I make these users appear in GAL?
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Extend your on-premises AD schema with the Exchange attributes, then you will be able to make the changes. Alternatively, you can disable dirsync, make the changes, re-enable it.

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