Notification Service not working over 4G connection

I use the Notification Service to send a notification to my mobile device whenever a window/door stays opened.
It works fine when my mobile device is connected to same network with Wifi. However I do not receive the notification over 4G connection.

I have access to my home assistnat server over 4G using duckdns.

What should I do to be able to receive these notifications when I’m away from home ?