Blow me! Working ok for me too. Tried a dozen times. Failed twice. Must be a glitch occasionally when you send a remote command. Also doesn't seem to like commands in quick succession. Says remote something or other in progress. Try again later.I did some tests this morning. Thanks to @GMat for his pointer:
So indeed, it does work without being plugged in. The "HVAC Settings" page is just to adjust the settings in the app, it doesn't send anything to the car. The "Start air con" is the only button to use. If you click the "hide HVAC pop up" checkbox in he "HVAC Settings", the "Start air con" sends whatever you set in the app to the car, otherwise it shows that screen to allow adjusting those settings before sending them to the car.
It works even if the car is not plugged in (the car is very silent compared to the Renault Zoe which is why I thought it wasn't working). The first of the 3 lights on top of the console does light up and flash (but if the windscreen is misty you won't be able to see it anyway). At least setting the temperature and defrost the windscreen work. "Heating" (of back window and mirrors I think based on the icons) does not - maybe that requires being plugged in, I still need to test that.
After about 15 minutes it stops by itself.