I'll link a couple of Apple's support documents on the subject here for you, starting with the Messages User Guide.
Messages User Guide for Mac - Apple Support
The key takeaways there are:
- Make sure that each of your devices is signed in with the same Apple Account as your iPhone.
- Make sure that your iPhone is turned on and connected to a Wi-Fi or cellular network.
- Set up Messages in iCloud (or) Text Message Forwarding, as described in the links below.
Set up iCloud for Messages on all your devices - Apple Support
Apple says when you set up Messages in iCloud on your devices, your entire messages history is kept up to date across those devices, even when you set up a new device. This includes messages sent or received with SMS, MMS, RCS, or iMessage. You don't need to set up Text Message Forwarding separately, because that feature is built into Messages in iCloud.
If you prefer to use Text Message Forwarding from your iPhone, guidance for setting up your other device is here:
Forward text messages from your iPhone to other devices - Apple Support
To set up the iPhone for Text Forwarding, do this:
Go to Settings > Apps > Messages.
Tap Text Message Forwarding. (If you don't see it, turn iMessage off and back on, then tap Send & Receive and sign in again.)
Tap to turn on each device that you want to allow to send and receive messages from this iPhone.
To use Text Forwarding on the Mac do this:
Open the Messages app. Then, from the menu bar, choose Messages > Settings... (or Preferences). Click iMessage and make sure that the Apple Account shown in this window is the same as on your iPhone. If it isn't, click Sign Out, then sign in with the correct Apple Account.
To use Text Forwarding on the iPad, do this:
Go to Settings > Apps > Messages and make sure that iMessage is turned on. Tap Send & Receive, then make sure that the Apple Account shown at the bottom of this screen is the same as on your iPhone. If it isn't, tap it to sign out, then sign in with the correct Apple Account.
If you still need more help, visit the Messages Support website.