Can not receive WhatsApp messages unless I open the app I have a Huawei P20 Lite with Android 9, EMUI 9.1.0 and WhatsApp 2.19.253, which is the latest version.

As I wrote in the title, I can't receive WhatsApp messages, after many hours of researching I didn't find any solution for my problem. This is the first time I face whit this problem, because I managed and configured variues P20 Lite but none of them gave me that headache.

The things which I did:


Turn on every single option for WhatsApp's notifications;
Uninstall the application and install it again;
Remove the app from the battery optimization process;
Clear the cache;
Enabled automatic start, secondary start and running in background, following this steps Settings -> Battery -> app launch -> 3 vertical dots -> manage manually;
No limit for internet traffic and allow background data, following Settings -> networks and wireless -> data usage -> select WhatsApp -> enabled the varius option.


Any idea or solution for this problem?

EDIT

When I send a message from my phone to this P20 Lite, on the message appears only 1 tick, such as the P20 is disconnected from wifi/mobile data, but the wifi is enabled and connected, the same behaviour with mobile data.
I just had this issue here with P10 - WhatsApp would not receive until opened or at least running in the background. Now it receives even if closed. I solved it by:
Settings &gt; Search for setting &gt; Ignore battery optimisation
If by 'Allowed' apps you don't see Whatsapp, click on the corner triangle for 'All apps', search for Whatsapp and 'Allow'