Push notifications while on mobile network

Hi everyone, I am unable to get push notifications while using mobile network.

For example, when the Whatsapp app is closed, I don't get any notification about new messages, yet if I do get the app open, I recieve them regularly. It is the same whit a few other apps.

For the record, the mobile network is working well. The problem is not there when I am connected to Wi-Fi.

*Push notification settings are OKAY as well.

Any suggestions?

Thanks.