Why do iPad SMS / MMS fail to send? CONTEXT

An iPhone and iPad are both:


outfitted with a T-Mobile SIM card + Data
configured with the same AppleID and 
presumably this ID is one's iMessage ID. 
IOS 12.X 


As I understand it:


Blue Messages are encrypted iMessages
Green messages are unencrypted MMS / SMS


OBSERVATIONS

iPad message app indicates:


iMessages are successful sent / received from and iPad's message app.
iMessages are synced to iPhone
green SMS / MMS fail to send: indication = "Not Delivered"


iPhone:


iMessages /SMS / MMS function as expected


Goal = send SMS / MMS from the iPad.

Any diagnostic question, posted as a comment, is appreciated.  Actionable guidance is sought.
Do you have text message forwarding enabled on your iPhone and iPad? If you go into the Messages settings on your iPhone, you will see some settings that you will need to enable in order to send SMS/MMS. Your iMessages are working because that only requires an Apple ID but you need to allow your iPad to send SMS/MMS via your cellular account from your iPhone. Good luck!