iMessage different on iPad then phone
iPads can send and receive iMessages through your phone number because your iPhone associates its number to your iCloud account. According to Apple's troubleshooting steps and support forums, the first thing to try is to repeat the association process in case that's somehow come undone:


Toggle iMessage off (Settings -> Messages) on both your iPad and iPhone. 
Wait a few minutes. Some guides say restart your iPhone, which can't hurt. 
On your iPhone, toggle iMessage on. If you see blue text below the toggle that says "Use your Apple ID for iMessage", tap it to sign in. 
Once your iPhone has activated iMessage and lists both your phone number and email addresses in (Settings -> Messages -> Send &amp; Receive), toggle iMessage on in your iPad's Settings. Your phone number should now be one of the "Send &amp; Receive" options on your iPad. 


Note that receiving and sending non-iMessage SMS texts on your iPad is set up separately.