Can I update with no WiFi

No. You cannot update iOS without Wi-Fi

Wi-Fi connection to the Internet is a requirement.

Or connect to a computer with Wi-Fi connection.

It might be possible but Only via connecting to a Computer that is running iTunes and via this connection update the iPhone 8 to newer iOS. Otherwise, the iPhone can not do an Over The Air( OTA ) update without Wifi

Yes, you can do it. Using data cellular, you have only to advance the day in the iPhone. For example, one month, 1 month 1/2.