I going to venture to say that the HUGE majority of people with this issue are having a Cellular Signal Strength issue. Period. I have spent more hours than I can count working with AT&T (and they are not alone with this issue) and Apple.
Sara, you MUST have a different issue on YOUR phone as you have full Cellular Bars (wifi is meaningless for this issue as Lawrence says) and another phone on the same carrier in same location works.
So it is either a setting that was missed - and many have been mentioned here to check. I have see oddities on some things "similar" not working when Location Services were turned off, for example. Not at all suggesting that is the issue here, Sara, but worth checking.
I'd bet folding money it is not an IOS update issue as there would be threads and threads on other forums I visit daily.
So this is a different issue than most on your specific phone.
Not sure if every setting has been posted here but likely most. If that does not help, go to Apple. If they do not help, have them exchange the phone. If that does not work, go back to Android and we will try to send you pictures :)
Cannot think of anything else to suggest. Good luck!