I have a fitbit versa watch linked to my dexcom and I use the glance app. (There was an optional $10 fee to buy). It relies on getting data via the clarity app in the cloud so the watch needs access to the phone and the phone needs access to the internet. (Really annoying when you are on a walk out of mobile phone reception because it no longer works). So yes you'll need mobile data on your phone (or access to wifi at home).