Proximity unlocking does not rely on LTE, and still works. Remote unlocking using the app would not work, because it relies on LTE. So the car can still be unlocked and used as normal in areas where there is no cellular reception. This is not some egregious oversight.
I may be incorrect, but I believe once you are near the car that it can connect via Bluetooth and operate over that local network, rather than going through the public internet and routing commands through Tesla's servers, but again, I could be misunderstanding how those features operate.