Add mobile number to iPad for iMessage
Is there a way to add my mobile phone number to iMessage on my iPad? It seems some iMessages don't make it to my iPad.
If you have an iPhone, you can use Continuity to get SMS messages on your iPad as well:
Use Continuity to connect your Mac, iPhone, iPad, iPod touch, and Apple Watch - Apple Support
If you don't have an iPhone, your options would be other types of messaging apps.