Your issue isn't absolutely clear. Do you have an iPhone - or just an iPad?
If exchanging messages with Android or non-Apple devices from iPad, you’ll need to explicitly enable Cellular SMS/MMS messaging for the iPad from your iPhone.
Continuity
Use Continuity to connect your Mac, iPhone, iPad, iPod touch and Apple Watch – Apple Support
SMS/MMS Messaging
How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac - Apple Support
On your iPhone:
Settings > Messages >
- Text Message Forwarding - set to ON - and explicitly enable your iPad from the list of devices.
- MMS Messaging - optionally set to ON if you wish to use Cellular picture messaging (additional messaging fees may apply).
Also ensure that Messages in iCloud is enabled on both your iPhone and iPad:
Settings > [Your Name / AppleID] > Messages - set to ON