not getting call or messages when my WhatsApp app is not ON

I am having an iPhone 6 S plus, bought more than three years back, working absolutely alright.

Since about a week I am not getting call or messages when my WhatsApp or Facebook apps are not ON, though I am always connected to internet. However, the moment I open WA, I get missed call, messages notifications. I believe, it has happened after I updated Apps or iOS last time (about a week back).

I have checked all possible 'unable' settings, all are enabled. Made Off / On several times also.