My understanding of wifi assist is that if there's no internet, even with a strong wifi connection, it will switch to cellular. Not perfect or 100% reliable. But admittedly, this may not be a viable solution to the problem. Also, there does seem to be conflicting information out there as to whether it will switch when having a strong wifi connection but no internet access so I could very will be wrong.

 

I'm telling you that if you have a strong wifi connection, it does NOT switch to cellular. I think your understanding is wrong. What happens is that your phone simply says "Cannot connect'.

 

I've never had an issue.
I don't use a phone when my car is running and my wife's iPhone and/or Samsung phone wi-fi is shutoff when in the vehicle and both connect fine with cellular.

 

If the vehicle is shut down, there will be no vehicle wifi, and your phones will not connect to wifi from it. With no wifi available, your phone will go to cellular.