We have to understand its limit to drive on the REX for 400 miles. The first limit is the power of the REX. The power of the REX is able to furnish enough energy on flat ground with no headwind at around 90 km/h. So if we are going up a long slope, or if we drive at 110 km/h, we will consume more electricity than what REX is able to furnish.
Meaning that at one point we will won't be able to sustain that speed or slope.
As an example, if the REX starts at 6% of the battery remaining, we will see the battery going down from 6% at that speed or slope. The available energy is the addition of what is left in the battery, plus what the REX is able to furnish.
How can we do it? Well, if we have the hack ( North America) or the European menu to start the REX at 75%, then it gives us a lot of reserves to slowly use until we are stuck at a lower speed or slope. Starting the REX at 75%, or even 25%, will give us a lot of room ( mileage at a speed or slope ) until the battery is too empty to sustain the desired speed, slope, or acceleration. If we do not have the menu to start the REX at 75% or less, then we have to limit our speed, slope, and acceleration to what the REX is able to furnish as energy.
The other limit is Ethanol poisoned fuel. If we are in a country that mandates a lot of Ethanol in the fuel, we will have from time to time some problems with the REX. I don't know about the latest version, but the first years REX and its fuel system did not like ethanol. I am in Canada, and tried to use the highest octane from Petro. Had a lot of problems. This high-octane fuel has a good percentage of ethanol, so they have to mix a lot of additives to achieve high octane rating. Looks to me that these additives make the fuel less stable when sitting for a month or two. The REX and its fuel system didn't like it. When i switched to Shell ethanol-free fuel, I didn't have a problem from this moment. Now it is no more available since a few months, so i will continue with Shell and see what happens with their new ethanol-mixed fuel. My remarks on fuel is not really pertinent to your question about the limit of using the REX on a trip. The first part is more relevant, the part about adjusting your speed and slope to sustain what the REX is able to furnish vs the reserve of battery before the REX starts
The REX is not able to sustain a battery level for slope or higher speed than 90 km/h. we have to manage our energy reserve to consumption: reserve in the battery plus what the REX is able to give