You can't "start" the car remotely. The only things you can do remotely are start/stop climate control and window defrosting, lock/unlock doors, and begin/end charging.
If the key fob's battery dies, there's a tiny little slot underneath the left side of the door handle that you use to pry open a cover and expose a key hole. You can then use the key to get into the car.
https://www.youtube.com/watch?v=OECOoNYZXmk
Once in the car, then you have to hold the fob up to the bottom right side of the steering column (there's an icon printed there) while pressing the start button. The car should then switch on.
For more details, check the manual.
The key fob's battery seems to die pretty quickly — in under a year for me. So I've bought some extra batteries that I keep in the glove box