Hi @frupy,
Welcome to the community!
Is it just for these two applications that you do not receive notifications for? If that is the case it's not impossible that this is related to a specific setting in these specific applications. Since these apps are from third party developers it's hard for me to say if this is related to a setting in the app or not, a suggestion is to contact the app developers to check this.
It could also be related to a power saving app or feature. Have you downloaded any power saving app? If so, try to (temporarily) uninstall these to see if there is any change.
I would also like to know if during this period of about five hours, are you using your phone? If not, it could be Google Doze being active that causes this. You can add these apps as exceptions to Doze under Settings > Apps > Tap the three dots > Battery optimisation > Apps > Mark the checkboxes next to Whatsapp and Viber.
It might also be a good idea to check if the STAMINA mode is active. If it is and you are not using your phone for a longe period of time you will not receive notifications from certain apps as performance is limited while STAMINA is active. You can find the option to disable STAMINA under Settings > Battery > STAMINA.
Let me know if this helped!
