You meed to verify that you have explicitly enabled 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.