If your iMessage has stopped working after installing your eSIM, there are a few things you can try to troubleshoot the issue.
eSIM Activation - Make sure your eSIM is activated and working properly.
Primary SIM Settings - Ensure that you still have your Primary SIM set to be used as the Default Voice Line and that the eSIM is set to be used for Mobile Data.
Check your iMessage settings - Make sure that iMessage is enabled and that your phone number and email address are listed correctly. Go to Settings > Messages and check the settings under Send & Receive.
Restart your device - Sometimes a simple restart can fix issues with iMessage. Turn your device off, wait a few seconds and then turn it back on again.
Reset your network settings - Go to Settings > General > Reset > Reset Network Settings. This will reset your Wi-Fi networks and passwords, cellular settings, and VPN and APN settings.
Update your device - Make sure that your device is running the latest version of iOS or iPadOS. Go to Settings > General > Software Update to check for updates.
If none of these steps resolve the issue contact Apple Support for further assistance.