You can just save your QR code as a picture on your phone, you don’t need internet.
This. OP says they won't have internet and they don't seem to fly often. Since most Airline apps are shitty, don't rely on their app. It could disappear; crash; forget who you are; know who you are, but forget about your flight. This never happens to frequent flyers with the regular airline, but it could totally happen to OP. (And no, it's not the super-low-cost budget airlines that have shitty apps)