On the latest version of the PHEV Remote Control app, under Settings, there is Wireless LAN Communication Settings, where you can change this. See screen grab. Not sure if it’s in the car head unit dialog somewhere. The default is OFF, i.e. Lan is ON when car is OFF and vice versa. This works for me - turn on heating and charging from in the house when the car is off, phone runs on 4G when in the car. Network definition on the phone for the car Wi-Fi is not to auto join.
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