A workaround is to have a second iPhone and run Zwift Companion on that. Connect this second iPhone to your first iPhone via its Wi-Fi hotspot. Connect your laptop on which you run the Zwift game to the same Wi-Fi hotspot on the first IPhone. Active mode of the Zwift Companion app should turn on automatically as soon as you log into the Zwift game on your laptop.
I also disable cellular data on my second iPhone to ensure that it is only connecting to the Wi-Fi hotspot of the first iPhone.