Sometimes when you're switching between SIMs on an iPhone or iPad, your iMessage settings need to be reset in order to work properly.
- Tap "Settings"
- Tap "Messages"
- Toggle "iMessage" to OFF and wait a second
- Toggle "iMessage" back to ON
- You'll see it says "Waiting for activation"
- Tap "Send & Receive"
- Make sure your email address is checked off
Selecting your email address will allow you to continue to send and receive iMessages using your brand-new eSIM data connection, without relying on your phone number.
When you're ready to switch back to your primary SIM, you'll want to go back (Tap "Settings" then "Messages" then "Send & Receive) to check off your phone number again.