I have the same issue, no recent FW updates according to other forums this is a worldwide issue for Lollipop users mostly S6
Samsung recently released an update to address this issue. Just go to Samsung Apps/Galaxy Apps store and download QuickPanel Restore. Once the mobile data icon has been restored, the app will prompt you to uninstall it which you can safely do so.
Hope this helps.
Thanks for the info. Missing icons in the Quick Panel already restored.