Like most on this topic, I've had a repeating disconnection every time I drove past a particular location. I tried various settings (ask to join wireless networks, location services, etc.) one at a time to see what would work. Finally, I turned off data roaming and it no longer disconnects at this location. I lose cellular data, though, since I'm no longer roaming, but CarPlay hasn't disconnected since I changed this setting.
I hope this helps others. I suspect that roaming is one of the factors involved, and the reason not everyone is affected at the same location could be due to having different carriers (hence different roaming locations).