How to Unblock a Mobile Device in Exchange Server

When a mobile device is being blocked by Exchange ActiveSync, there are two ways that you can unblock it. The first is using the Exchange admin center for Exchange 2013/2016. Navigate to the mailboxes view, select the mailbox in question, and in the right-hand pane click the View details link under Mobile Devices. The list of mobile devices associated with at user will be displayed, and you can highlight the device you want to unblock and click the Allow button, then save the change.
exchange-activesync-unblock-device
This change will add the device to the user's allowed devices list. If you'd prefer to just remove the device association, click the delete button instead.
The other method is to use the Exchange Management Shell. Run the Get-CASMailbox cmdlet to see the blocked device IDs for the user.
To add a blocked device ID to the allowed device ID list, use the Set-CASMailbox cmdlet.
Alternatively, you can just remove it from the blocked device ID list.
Or, if you'd prefer to just remove the device association, use Remove-MobileDevice.