Not sure if this is just my hardware or what, but I have an iPhone X and just updated to IOS 11.3 yesterday. Today, for no random reason, my phone was showing no service. All other iphones in my household, not on IOS 11.3, had ATT service. Tried turning on and off airplane mode several times but no go. Had to reboot phone before cell service was recognized again. Just posting this in case anyone else has this random occurrence after updating to IOS 11.3.