How to use Unlimited text plan on iPhone with no data

Hi, daughter just got unlimited talk and text plan for her iPhone. She also has an iPad and uses wifi - at home - for both her iPhone and/or iPad. What does she need to do to be able to “text from anywhere” on her iPhone when she is not connected to wifi? Messages are not being received on her phone when we are out, but only received on her iPad that is sitting at home connected to wifi…. Does it have to do with Cellular Data need to be turned on her phone?? I thought it would automatically switch for her from SMS to iMessage based on if she was connected to wifi but doesn’t seem to be the case