Phone auto go into silent mode : no more notification sounds when new Messages come

I noticed usually  at night (though I'm uncertain at time), my Note4 (which I bought as
a used device) will go into 'silent mode' ie when new Whatsapp/SMS comes, it is quiet
despite that on some crucial Whatsapp group, I set 'Custom notifications" to alert
with sounds (as I am on-call 24x7).

I will notice that following will become disabled (ie at the lowest/minimum setting)
& after enabling it back, it will go back again & again (with each day) without
intervention from me.
  Settings => Sounds & Notifications => Notifications

When new Whatsapp comes, the pop-up still appear tho
(this is a setting under  Custom Notifications)

When I first got the device from the seller, there's a pop-up saying "Good Night,
do you want to silent your phone" which I will cancel but I don't see this pop-up
anymore the last 2 weeks.

What can I do to disable this 'auto-silent' mode which doesn't reverse itself
even day breaks (it's morning)?
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It sounds as though Do Not Disturb has been set according to a schedule. Go to Settings -> Sounds & notification -> Do not disturb -> Automatic rules and check that nothing has been set up in there.
☠ MASQ ☠Commented:
Check your settings under Do Not Disturb Mode - I think on the Note4 it's called Blocking Mode

Drag down the menu from the top of the screen and press the grid on the top right corner to get at the system settings.

Look for Do Not Disturb/Blocking Mode
If it's enabled try turning it off to start with and if this fixes it go back in and adjust the settings until it does what you want.

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sunhuxAuthor Commented:
Both of your suggestions  arrive at the "Do Not Disturb" via different ways:
currently in both cases, "Do Not Disturb" is not turned on:
I'll monitor 2 more days;  for 2 days this did not happen tho I don't recall I change anything.

Will update in 2 days' time
sunhuxAuthor Commented:
The issue does not surface so far, so I'll clsoe this
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