An iPad, whether a Cellular model or not, is not capable of directly accessing Cellular Voice or Messaging features. An iPad with Cellular connectivity alone can only access IP-data services. An iPad is not, and can never be, a standalone substitute for an iPhone.
However, If paired with an iPhone, the capabilities of iPad significantly broaden to include access to Cellular Voice and Messaging services of the associated iPhone - relayed to the iPad over WiFi using Apple’s continuity features. For the most part, an iPad using Continuity is able to access Cellular calling features, of the iPhone, when both the iPad an iPhone are in close physical proximity and are connected to the same WiFi network - but this is not the end of the story.
More information about Continuity and the relevant Cellular services can be found here:
Continuity
https://support.apple.com/en-gb/HT204681
SMS/MMS Messaging
https://support.apple.com/en-us/HT208386
WiFi Calling
https://support.apple.com/en-gb/HT203032
If you do have an iPhone, you must explicitly 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
More information:
How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac - Apple Support
Fault finding if needed:
If you can't send or receive messages on your iPhone, iPad, or iPod touch - Apple Support
I hope you find this to helpful in resolving the issue.