Doesn't Waymo use a similar approach except that you don't own the car?
I dunno. Do they? How does it cope when there’s no reception?
You just have to get the wired version if you expect to go through tunnels.
That will be a looooooooooooooong cable :)
EDIT: (or a tether, kinda like the one skydivers have for when jumping off a plane with the rope being tied to them chutes, and deploys them as they exit from the back)