Your iPad receiving text messages is not an indication of your iPad directly using a Cellular connection - as text messages received by your iPhone may relayed to your iPad using Apple’s Continuity features.
Information about Continuity and the relevant Cellular services can be found here:
Continuity
Use Continuity to connect your Mac, iPhone, iPad, iPod touch and Apple Watch – Apple Support
SMS/MMS Messaging
How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac - Apple Support
WiFi Calling
Make a call with Wi-Fi Calling – Apple Support
If you have an iPhone - and both iPad and iPhone are signed-in to iCloud with the same AppleID - to send SMS/MMS messages (green bubbles) from your iPad you simply enable Text Message Forwarding for your iPad on your iPhone...
On your iPhone:
Settings > Messages > Text Message Forwarding - set to ON - and explicitly enable your iPad from the list of devices.
Also ensure that Messages in iCloud is enabled on both your iPhone and iPad:
Settings > [Your Name / AppleID] > Messages - set to ON
If your iPad is using your iPhone’s WiFi hotspot, instead of seeing the WiFi connection symbol at top-right of the screen, you will instead see the chain icon - that shows a “link” (chain-link) icon. All the status icons are described within the iPad User Guide; here is a link to the relevant page:
Learn the meaning of the iPad status icons - Apple Support