You need to do a couple of things.
On the iPhone, you need to use your AppleID for iMessages, AND associate that phone number with iMessages and that AppleID.
The iPad (and your Mac) can sign into iMessages using that AppleID and should now see any iMessages sent to the cell number.
(Settings>Messages>iMessage enabled)
{Settings>Messages>Send & Receive)
The final step is for the iPhone to "forward" SMS/Text messages to the iPad (and Mac). Then they can see texts from non-iMessage users.
(Settings>Messages>Text Message Forwarding).