I had that exact same issue with my iPhone and J1.2. The non intuitive fix that is not written anywhere is to **not** manually connect to the WiFi hotspot of the car ever.
It means, in WiFi settings of your iPhone go and “forget” the network (taycan_something), and never manually join the WiFi hotspot ever or try to touch any of the settings.
I ended up in that situation by manually connecting to the hotspot to try to “fix” CarPlay. But it only made it worst (no more internet in my iPhone as you described).
CarPlay works by using Bluetooth first and then the car / iPhone establish direct WiFi connection which the iPhone treat differently than you manually joining the hotspot (it won’t use the connection as a route for internet).
Something to also NOT do is untick “auto-join”, it will break wireless CarPlay because the iPhone won’t be able to create the WiFi direct connection anymore.
So:
1- Don’t manually join the hot spot ever in an attempt to fix a non working wireless CarPlay.
2- If you did, forget the network in your iPhone settings.
3- Never untick auto-join in an attempt to fix 1. or you’ll break CarPlay too.
Yes it’s massive trash and undocumented. I only managed to understand that by founding an obscure Reddit thread I can’t find anymore where someone enabled debug logs on their iPhone as developer and was able to read what CarPlay was trying to do and why it was failing. Could you imagine if these errors were visible to the customer. This is a much on Porsche as it’s on Apple.
Since understanding this, my wireless CarPlay has been reliable enough (still slow sometimes but it eventually works without me messing up with any of the car or iPhone settings).
