How do I use my iPhone cellular service on my iPad
How do I Setup cellular service on iPad to use IPhone plan
iPad Pro 10.5-inch, Wi-Fi, Cellular
If you have a cellular (not wifi only) iPad and your wireless provider supports
iPad data plans, ask your wireless provider for the appropriate
data only sim. Be advised there may be a monthly charge for this service.
If you are trying to use your iPhone as a hotspot to provide wifi
access for your iPad, you need to see if your wireless provider offers
that and if your account is properly provisioned (may also be a charge).
Here is a link to an Apple article on setting up a HotSpot: