I cant use my cellular data

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

See also -> If you can't make or receive calls on your iPhone

If you can't make or receive calls on your iPhone - Apple Support

Contact your carrier.

See if anything here helps. 

About cellular data settings and usage on your iPhone and iPad

Activate cellular data service on your iPad Pro, iPad Air 2, and iPad mini 3 or later