It may work. It may not work. WiFi is spotty, overloaded and unreliable. What it can't do is be guaranteed to work.
But, if I had to bet I'd bet on being able to get a signal working long enough to request a ride.
One other thing to consider, I've had several drivers call me, or text me. Usually call though, to check in about where to meet. I always text them with my exact spot, since obviously there are lots of people around and it's not always easy to spot who needs picking up.
Maybe the drivers here can chime in on how they'd handle it if they wanted to call a passenger and they aren't using a phone. If there is no phone on your device. I know my Mini can't be a phone but I do think there could be some sort of app that mimics a phone but have never tried one to know if they work or not. Just something to consider.