Hi,
in the current ARM version (2019.4.2.4), the Active Sync function cannot be managed on existing mailboxes.
This function is missing in the ARM tool
You can only turn on the option when you create a new mailbox.
The PowerShell command:
Set-CASMailbox -Identity "mailadress" -ActiveSyncEnabled $false
Set-CASMailbox -Identity "mailadress" -ActiveSyncEnabled $true