Thx for the helpful comments.
Also stumbled across another trick that appears to work - GNI has a "Wi-Fi sleep policy" in Advanced Wi-Fi Settings which determines when the GNI switches from accessing data via Wi-Fi to accessing data via cell network. One of the "Wi-Fi sleep policy" options is "never", i.e. if this option is selected, it appears the GNI will never switch from getting data via Wi-Fi to getting it via cell network, thereby (presumably) eliminating the risk that you'll run up big data download bills when you don't have a monthly data plan. On those rare occasions I want to get data via cell network, I can just change the option to something other than "never".
Only thing I'm not sure about is how it works when you're out of Wifi range, i.e. does "never" mean "never", even if there is no Wifi signal.