I have an iPhone 11 Pro (I know, I know

) and no subscriptions. I have an unlimited data plan and just use that. No need for anything else as far as I know to get remote start, locking, etc. to work and to use Carplay.
The only thing I do notice is that after using remote start, I see the weird "WiFi connection interrupted, try again" message. I hit retry, and then it goes away. If it reconnects then Carplay works fine (after about a 10 second pause..assuming it's reconnecting), if not, I go into the settings for my profile and enable the "use for Carplay" slider and all is fine.
Not sure if this helps anyone, but I was stuck on "why can't I use carplay after remote starting" and figured out that it seems like if that wifi connection is lost and doesn't reconnect you have to get into your settings and re-enable it.