Go to settings then Cellurlar Data and make sure Cellular Data is ON. Under settings, go to Wi-Fi and turn Wifi off so the iPad will be forced to use 3G/ 4G LTE. If you set up a data plan it should work otherwise I would call AT&T or go to a store in person but I'm pretty sure that you have cellular data turned off. I don't have a data plan but when I turn off wifi the "3G" icon shows up because I have cellular data turned on to be able to use the find my iPhone app anywhere. If cellular data is turned off your iPad will not be able to be located unless you're on wifi.