How to connect cellular on iPad

Assuming that you have a WiFi+Cellular model iPad, this support page describes activation of cellular service:

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

If instead you have a WiFi Only model of iPad, you can connect to a mobile phone hotspot. For iPhone, or another iPad with cellular service:

If Personal Hotspot is not working on your iPhone or iPad (Wi-Fi + Cellular) - Apple Support

You can also use a Mobile Broadband router (a.k.a. MiFi). These are available from good computer stores and Cellular Carriers.