Cellular phone reliability is pretty low as you can travel and find dead spots. This has nothing to do with the make of the phone.

Given your words here your employer should set you up with a satellite phone. Those work where cell phones don't and there are other considerations but given what you wrote, it's the only way today.

I worry also about your question of support. Almost have gone to pay as you go support. It's rarely included with your Apple experience being the exception.