Is it possible to update iPhone without using Wi-Fi?

If you have an iPhone 12 or higher and your carrier is providing a 5G signal and your phone is set up to use 5G, you can update using cellular data.

Otherwise, you need a good Wi-Fi connection to update.

Additionally, you CAN update your phone sans WiFi with a cable and a computer, BUT that computer WILL need solid internet connectivity.

See:

if you're using a Mac with macOS Catalina or later

or 

if you're using a Mac with macOS Mojave or earlier or a Windows PC.

You will need to use Wi-Fi or connect the iPhone 8 to a computer to update. The iPhone 8 will update to iOS 16.7.8 when you run the process.