Why is my cellular data not working?

Try 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

• Contact your carrier
• If your carrier confirms that there's no issue with your account or network, but you still can't connect to a cellular network, contact Apple Support.

See also these Apple articles below:

Manually update your carrier settings on your iPhone or iPad

Manually update your carrier settings on your iPhone or iPad - Apple Support

Update your network provider settings manually on your iPhone or iPad

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