Messages

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.

Go through the steps in here and make sure that you have everything still set up in order to forward SMS messages from your phone to your iPad.

How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac - Apple Support