If you take your phone near the car, do you have good signal levels? While coverage with one carrier doesn't necessarily apply to others, it's a start. IF your signal strength is low, that in the car might be low, too. Have you tried it in other places?
Mine seems to work about 80% of the time. I know my coverage where the car is parked is marginal sometimes.
The app indicates the two steps where it first communicates with the server, then the server tries to talk to the car...does it ever get to the last step?
FWIW, I couldn't talk to mine yesterday...haven't tried today.
Some of the original s/w releases in the car had issues, and that portion of the car's logic could get hung up and never respond. At the time prior to them updating the car's software, you could get it working by either unplugging the module and replugging it in, or pulling the fuse (easier) and replacing it. That hasn't been true now for at least a few years.