I kept airplane mode on for my Note 5 while I traveled in Asia for a whole month few years ago. I could turn on/off wifi at my friends' homes and use public wifi. I never used any data from my wireless carrier and my bill and logs reflected that. IDK if android or ios updated things, but I suspect not. Airplane mode should be a killswitch for all wireless traffic. Something else happened here.
Also, international free data from T-Mo is 2G speed. That is SHIT SLOW. I can barely use email and forget it if there's a photo or file attachment. 1.8 GB on 2G would take FOREVER. And I seriously doubt cruising around in an airplane would be a stable enough connection wherever the phone was getting internet from. (even southwest free inflight wifi is hiccupy.)
Also, international free data from T-Mo is 2G speed. That is SHIT SLOW. I can barely use email and forget it if there's a photo or file attachment. 1.8 GB on 2G would take FOREVER. And I seriously doubt cruising around in an airplane would be a stable enough connection wherever the phone was getting internet from. (even southwest free inflight wifi is hiccupy.)