How to get cellular data without a sim tray?

The other option is Mi-Fi device from your carrier. These connect a cellular network, and can provide the iPad with a Wi-fi connection it can use to access the internet when away form traditional Wi-Fi.

You can do a personal hotspot from your phone, but if your iPad doesn’t have a sim tray, it won’t have cellular antennas either of course, so you won’t be able to get cellular directly on it.