So you are saying the SuperChargers aren't already on a network? How does your car and your app know how many supercharging slots are available and how many are broken?
They are clearly already networked, it would just be a question of upgrading bandwidth, if necessary, and separating off a guest wi-fi network, which is child's play for the network teams.
Cellular - both in the car and on the Superchargers. You can see a cell antenna on each V2 supercharger. V3 sites seem to have a common cell modem for the entire site. It appears that some V3 sites might be getting hardwired in areas where that is convenient.