Cellular roaming

Does your iPad have a large plastic piece on the back on the end by the power button? If not, it’s not a WiFi+cellular device as the plastic cover over the antennae is very obvious in iPads with a cellular radio.

Go to Settings > Cellular. If you do not have Cellular there, then your iPad does not have cellular feature. It is only Wi-Fi compliant!

Perhaps you have WiFi Only iPad - and not a WiFi & Cellular model?

A WiFi Only model of iPad has no cellular hardware or capability. There will no Cellular Settings or SIM tray.