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!

Your iMac does not actually send SMS/MMS messages; your iPhone does. Your iMac is utilizing a feature called Continuity.

One of the requirements for continuity is that your device and the iPhone both be on the same WiFi network. Why your iPhone specifically? Well, SMS/MMS messages can only be sent over the cellular voice network (it’s not a protocol found on the cellular data network). Your iPad is data only and doesn’t have a messaging plan attached to it.

What you’re probably find is that:

• iMessage, will use the most efficient network to send a message; if the recipient is using another Apple device, it will go iMessage
• If your iPhone and iPad are on the same WiFi network, you’re probably using your phone for messaging anyway
• Since your iPad has two data connections - cellular and WiFi, one way or another it’s going to find a way to connect to the iMessage server before resorting to SMS.

If you want to force sending via SMS, you’ll need both devices on the signed into the same iCloud account, the iPad cell data disabled, and text message forwarding enabled to “trick” it into sending SMS via the iPhone.