Hi Wardts,
welcome to the Apple Community.
It seems only iPad models with Wi-Fi + Cellular can use Cellular text messaging.
However, you should be able to send text messages with Wi-Fi only in iMessage app..
Wondering:
which iPad model you are using,
which iOS is installed,
This article: "Set up Messages on iPad"
Says:
"In the Messages app,
"iMessage texts appear in blue bubbles, and SMS/MMS texts appear in green bubbles."
Complete article: https://support.apple.com/en-au/guide/ipad/ipad70482978/ipados
For more, try this article:
What is the difference between iMessage and SMS/MMS?
https://support.apple.com/en-au/HT207006
Hope this helps you :-)