Smart Limits-Data use off but now phone/text do not work? Says no service but I only want to stop da

I use smart limits to limit the times the kids can use data on their iphone 7s. However, I do not want to prevent them from using the phone to make a call or to text (understand imessage wont work). I tested on my phone (using ATT website instead of Smart Limit app) and turned data off on my phone and my phone shows "no service" which is odd because I am not turning off service. I only want to turn off data. How do we turn off data on teen's phone but allow then to still have phone/text service.

Is there a way to allow for data off but location still on? I realize that location may be fully reliable on data but hoping there is a way.