Annoying but not unexpected
We've seen issues like this several times over the past few years. Every Tesla owner should be aware that relying on the app to unlock and start the car isn't safe. If the car is in a spot that doesn't have coverage, you're out of luck. If the backend servers have problems, you're out of luck. If AT&T messes things up, you're out of luck. All of those have happened.
What most owners agree on is that the app should be able to use Bluetooth to talk to the car directly if you're right outside it.
Yes, it is annoying when you have software glitches, and doubly so when they are from a very expensive purchase. Tesla has had more than its share of glitches, but on the other hand, Tesla is much more aggressive about rolling out updates that benefit owners, so taken as a whole, they're still doing it right.