I own a 2014 REX upgraded with 120 battery. One trick is the 10L NATO canister (e.g https://www.amazon.com/Wavian-USA-JC0010RVS-Authentic-System/dp/B00VVFTEV4/?th=1) that fits in the frunk. So one first drives using all REX fuel, then at some point stops at a gas station, filling up the REX and also this canister (fill only up to 8.5L), and resume travel with less than 5minutes of stopping using REX again, and then almost all battery, getting to a fast-charger in a place with decent lunch (here you have to creatively plan in advance). Before leaving, fill up the REX from the canister. This will get me 750km with a single charging stop, and with a second fuel stop at the end to 850km.