9.0 pie - Notifications arrive when the app is opened, but not in the background

I don't know if it's the right place to ask my question, if not, sorry.

1st, I was using Sony Xperia Z5 premium for the last three years, never faced any issues but few days ago, I switched to Samsung galaxy s10+ and the problem I'm facing is, I'm not getting notifications from WhatsApp or some other apps when they're not opened. When I open them, I got couple of notifications. If I don't open that app for whole day, and at night when I open it, I get notifications of all day.

Please guide which settings or options I should see?