It’s a very common issue and easily fixed!
First, try to deactivate and reactivate iMessages on your phone. On your phone, tap Settings > Messages > iMessage and turn it off and on.
If your iMessages are still sent with your temporary number, verify if your settings are updated. Tap Settings > Messages > Send & Receive and verify if the correct mobile number is displayed under the START NEW CONVERSATIONS FROM section.
You can also try to reset network setting on your iPhone. Tap Settings > General > Reset > Reset Network Settings.
Please note that you might be charged by Apple for the activation requests sent to Apple Servers each time you activate / reactivate the iMessage function. For more details on iMessages settings, you can have a look at the dedicated Apple Support Platform by clicking here!