Today i'm all the day without internet, ISP said optical cable to my home is break. I have LTE internet via iPhone. Of course i don't want to use it for online racing, but maybe for practice. But my tariff plan with limited internet traffic. This why i ask about it.
Test it on your own. Start the game and go into SP race or practice session. Your phone should be able to show the traffic mobile teathering used. By the way, you can cut your connection when the track is loaded. And have an eye at auto-updates of other apps and steam-games. Could cost u a lot of traffic...
Thats why we need offline mode as not everyone has continuous connection 24/7. I havent been playing R3E for a while because my connection was down last week where it is impossible to connect to service.