You set the EnableMobilityparameter 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.
To fix this issue, install the May 2019 cumulative update 6.0.9319.548 for Skype for Business Server 2015, Web Components Server.
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.