You set the EnableMobility parameter to false in order not to let users use Skype for Business on their mobile phones in Microsoft Skype for Business Server 2015. In this scenario, the users can't sign in to the Skype for Business for Mac client automatically after switching a network or hibernating the computer. In addition, the Skype for Business for Mac client displays "Restoring the server connection..." but does not connect, and displays the sign-in screen.
As a workaround, run the following PowerShell command to turn on the Skype for Business mobility.
Set-CsMobilityPolicy -Identity "site:Redmond" - EnableMobility $True
In this example, mobility is enabled in the mobility policy assigned to the Redmond site. You do this by setting the EnableMobility property to True.