Aha! Found the solution. In order to get critical notifications to work, you have first to open More > Permissions > Critical Alerts and enable within the Homey app on iOS and then this setting becomes available in the normal place within Settings > Notifications > Homey in iOS itself.
If any devs are following along, it might be an idea to make this more visible to the user somehow when this is put into stable release. Provided the same feedback on TestFlight also.
