iPhone 7 recently quit connecting to cell service because it said it needed an IOS update

My iPhone 7 recently quit connecting to cell service because it said it needed an IOS update that supposedly couldn't be done from my phone even though i was current on all updates. this happened while i was on vacation and i ended up having to buy a new iPhone. well now i want to sell the iPhone 7 but need to get it working so someone else can use it. i have factory reset the phone and now in the startup screen i get a message saying my phone cannot be activated because an update is required and the only option it gives me is to shut down. HOW DO I FIX THIS?? i live in an area where I don't have an apple store available to me.

[Re-Titled by Moderator]