My iPhone 7 is showing that I have no service

hi my iphone 7 is showing that I have no service. It told me that I needed to update my phone and I did exactly that. After I updated it it worked for about 3 hours then it stopped working again. Now it keeps showing me that I need to update my phone again but when I go to update it, it just shows that there is no update available. What can I do to turn my cellular data back on?

[Re-Titled by Moderator]