Yes. You can use the "Mobile Data" button in the quick settings. Swipe down from the top and look for the Mobile Data button. If you don't see it click the edit (pencil) icon. This will expand the list. If you want it at the top so you don't have to expand the list each time just hold down on the icon and then drag it to the top row. Now you can just click that button to turn your mobile data on and off.
Go to device information, and the first drop down menu is to select a network. For AT&T, choose GSM Auto to switch off LTE. Remember the setting it was on, because that's the one you need to choose to bring LTE back. Or, reboot the phone to bring it back.
You guys must be on really old Lollipop firmwares as there has been a mobile data toggle in the notification bar on my last 3 Lollipop updates since February, on the international, G900F, model on stock Samsung firmware. See screenshot...
No. Vodafone U.K. on an unbranded G900F, international, model on stock Samsung firmware. I don't think any of the U.S. carrier branded, minority variants of the Galaxy S5 have a high enough changelist for this and other features. See post #'1 of Lollipop release. Massive update!!!
I have a Galaxy S3 and the Data On/Off app is great for making the quick switch to mobile data, but recently that same app on my wife's Galaxy S5 has stopped working all together. The screen widget icon no longer toggles the image and selecting the widget has no effect on the status of the mobile data settings like it used to. Now she is forced to go through the settings interface in order to turn on/off the mobile data options. I tried uninstalling/reinstalling the Data ON/Off app, but is still does not work on her phone. Same story for the Data Enabler Widget and the Mobile Data On/Off apps