You need to configure your internet/APN settings first.
Follow the instructions here:
My phone's internet is not working. Why?
In the link above there's also a note about iPhone hotspots that may be relevant due to the stupid way that Apple do things to make things awkward for some users on some networks...
Then follow the instructions here in addition to the aforementioned note:
https://support.apple.com/en-ie/HT204023
Hope this helps.
