Cannot send SMS messages from iPhone linked on Apple Account

I have three devices:

1. iPhone X (without SIM card)
2. iPhone 8 (with SIM card)
3. MacBook Pro

The situation is, I am using the iPhone 8 to receive texts and calls, and they are forwarded to my iPhone X, and by extension to my MacBook Pro since all three devices are linked together with the same Apple account. All three devices have access to wifi, but only the iPhone 8 has data (sim card).

Here's what's happening:

• I can receive calls and SMS/iMessages on the iPhone 8 naturally.
• I receive SMS/iMessages on all three devices.
• I cannot send SMS texts from the iPhone X, but I can send and receive them from the iPhone 8 & MacBook Pro. When trying to send SMS messages with the iPhone X, the message immediately has an error (!).
• I receive calls only to the iPhone 8. It used to show on all three devices, but that has since stopped occurring.
• The iPhone X is setup to receive forwarded calls and texts from the iPhone 8.

I would like to send and receive SMS messages on all three devices, if possible. I don't understand why I would be able to send SMS messages on my MacBook Pro (not even a phone) but not my iPhone X if they all have access to wifi and are logged in with the same Apple Account.

How do I send SMS with my iPhone X?

Let me know if this is clear and any help is appreciated.