I am facing the same problem on my iPhone 7 as well since yesterday. The error reads "Mobile Data Update Failed -Your iPhone cannot make and receive calls or access mobile data until it has been updated". I am already on ios12, not sure what to update.I have tried restarting my phone, used airplane mode on and off, ejected sim and reinserted, and even tried resetting my network through settings. Till last night, i would get lucky using one of these fixes but for the last 12 hours, my phone has been in No Service mode. It is showing a warning sign and no service instead of my carrier name in the status bar and also showing Error next to Phone in settings. On apple support page, it says if I try to restore my iPhone, I wont be able to activate my device. Kind of lost now so dropping a message here. Hope we can find a fix.