Hi there!
I hope besides your issue your doing well.
Assuming that your iCloud for the iPad and iPhone are the same please follow the steps below since your iPad may be blacklisted to display messages:
Settings > Messages > Text Message Forwarding - set to ON - and explicitly enable your iPad from the list of devices.
If your settings are done correctly and have tried again please also try restarting your devices.
On an iPad with a Home button: Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.
On other iPad models: Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the top button. When the Apple logo appears, release the button.
I would have recommended contacting your SIM provider as they may be interfering with this process (please note this is very unlikely).
Also please try this video and see if you have done the steps correctly:
*******
This video is not mine.
If none of these work. Please reply to me with the following:
- What iOS version you use (to make sure)
- If it is iMessage you are having the problem with or other apps (such as WhatsApp).
Stay Safe!
Have a great day.