How can I update my phone
Why’s my iPhone network not available and I can’t call or receive messages or data.
iPhone 7, iOS 14
You can update an iPhone 7 as far as the latest release of iOS 15. You cannot update it to iOS 16 or higher.
However, what you're complaining about is not being able to use your cellular network. That suggests that there is some issue with your cellular provider or with your cellular account. If you are being prompted to install a carrier settings update, see
Manually update your carrier settings on your iPhone or iPad - Apple Support
If you continue having problems with your cellular service, contact your carrier.