Sometimes when you're switching between SIMs on an iPhone or iPad, your iMessage settings need to be reset in order to work properly.

Here's how to reset iMessage:

• Tap "Settings"
• Go to "Messages" and toggle "iMessage" OFF
• Go to "FaceTime" and toggle "FaceTime" OFF
• Restart your iPhone
• Go back to "Settings"
• Go to "Messages" and toggle "iMessage" ON
• You'll see it says "Waiting for activation" 
• Tap "Send & Receive"
• Make sure your Apple ID email address is checked off
• Go to "FaceTime" and toggle "FaceTime" ON

Selecting your email address will allow you to continue to send and receive iMessages using your brand-new eSIM data connection, without relying only on your phone number.