Hi Kjk670,
Thanks for being apart of Apple Support Communities. It sounds like your iPhone has a cellular update available, but it won't allow you to complete the update, even using your computer. That would prompt us to seek a solution, as well. We'll do our best to assist.
Do you happen to get an error "Cellular Update Failed"? If so, try out these specific steps for this issue: If you see an Update Failed or Apple Pay Not Available alert after iOS update or restart under section, 'Cellular Update Failed'.
Note: You'll need access to a computer to accomplish these steps.
Should the error continue, then you'd want to bring the iPhone in for an inspection. This can help to locate a service center: Find Locations And our resource here can prepare you for the visit: Get your iPhone, iPad, or iPod touch ready for service Keep in mind, due to the current conditions, these stores may have limited hours or available appointments, they also may have requirements for the visit (For example, wearing a mask in the store).
Wishing you the best, take care!