This problem seems to appear due to disabling the mobile data icon in quick launch tiles. And there is no option to turn it on with the mobile device alone. And this problem seems to more common with Samsung devices. I had the same problem with my Samsung note 5 and I was able to restore the mobile data quick launch icon with this guide. This method uses the tool ADB(android debug bridge) to access mobile via computer and add the mobile data icon to the quick launch tiles. If tools like ADB are too technical for you simply use an app like Mobile Data Switch as an alternative to the mobile data quick launch icon.