My iPhone frequently loses connection for some reason. I can show 5 bars (AT&T) and LTE but will be unable to connect to the Internet or send text messages. I took if in to Apple and they did a "firmware erase and restore" and said if the problem persists to just flip it in airplane mode for a few seconds and then I should be fine. This does seem to work, but I'm not getting phone calls and text messages at times and I'm assuming its when its state of no connectivity. Could this be a SIM card issue or should I go back to Apple?