My data cellular data is turn off how do I fix it?

Try first to optimize the range of the mobile network, see this Apple article:

If you see No Service or Searching on your iPhone or iPad

If you see No Service or Searching on your iPhone or iPad - Apple Support

If you still don't have a 'service' after that, see -> iPhone 7 Repair Program for "No Service" Issues

iPhone 7 Repair Program for "No Service" Issues - Apple Support

See also these Apple articles below:

Manually update your carrier settings on your iPhone or iPad

https://support.apple.com/en-us/HT201270

Update your network provider settings manually on your iPhone or iPad

https://support.apple.com/en-gb/HT201270