Unless you also have an iPhone, your iPad cannot send or receive SMS or MMS messages to phone numbers - as these are Cellular Carrier features. Without an iPhone, your iPad is limited to sending/receiving messages via the Apple-proprietary Messages App.
Native iMessages are directed to Apple recipients by their registered email addresses - and not by their Cellular phone numbers.
To use SMS/MMS messaging from the iPad Messages App, your iPhone must be logged-in to iCloud with the same AppleID - and message forwarding to your iPad must be enabled on the iPhone.
https://support.apple.com/en-gb/HT208386
An iPad alone, whether a Cellular model or not, is not capable of accessing Cellular Voice or Messaging features. An iPad with Cellular connectivity alone can only access IP-data services.
When paired with an iPhone, the capabilities of iPad significantly broaden to include Cellular Voice and Messaging services of the iPhone - relayed to the iPad over WiFi using Apple’s continuity features.
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
I hope this is helpful in clarifying your iPads Messaging features.