How do I add cellular service to my iPad?

Do you have a Cellular capable iPad? iPad - Cellular - Apple

If so, follow the guidance provided in Set up cellular data service on your Wi-Fi + Cellular model iPad - Apple Support.

First, you need to have a WiFi+Cellular model of iPad. If instead you have a WiFi Only model, then you cannot add cellular capabilities to the iPad.

Set up cellular service on iPad (Wi-Fi + Cellular models) - Apple Support

Set up mobile data service on your Wi-Fi + Cellular model iPad – Apple Support

View or change cellular settings on iPad - Apple Support

If you do have a WiFi Only model, if you have an iPhone (or other mobile phone) with Hotspot capabilities included in your Cellular Plan, you can use the WiFi Hotspot feature to access cellular data from your iPad.

https://www.imore.com/how-to-tether-ipad-wi-fi-iphone-personal-hotspot

If you have an iPad wifi+cellular, contact your carrier and ask for a data plan using a SIM card, keeping in mind that no iPads can support phone calls, just data)

If your iPad is a wifi only model, you cannot add any cellular capability.

Regards

Giulio