smart TV ? or chrome cast receiver device ?
a smart tv, is an internet device just like a tablet etc..
as i understand it. a chrome stick is a receiver and talks to the casting app on your phone, but it does not connect to the internet wifi hotspot all by itself
blue tooth , cable or wifi ? its a one way cast to the chrome stick
it has to be on the "local Network" somewhere to receive the video data from the phone or tablet or PC, net book etc..
the important part IS the cellular network sees a device with streaming option in the plan, so that data is not counted in your case