No notifications about SMS arrival iPhone 7

Hello! I have problems with notifications about SMS arrival with my almost new iPhone 7.

I don't receive any notifications about arrived message if iPhone is in sleep - sometimes iPhone turns display on and playes "ping" and sometimes does not. The device is in front of me all the evening long - dark and silent - while I work on my computer. Few hours later I pick up phone and see a dozen of unanswered messages. No ANY notifications arrived during this evening. And sometimes everything is fine! iPhone informes me about SMS as it should.

Problem reveals itself mostly (but not limited!) at night. Beyond 10 PM.

I already:

1) Double and triple checked if my dnd function is turned off.

2) Double and triple checked if my "in car" function is turned off

3) Double and triple restored device from a backup

4) Double restored device with UDF mode.

5) Erased the phone if problem persists in new setup.

The problem remains. Does aanyone have ide what's wrong with it? I don't think its a hardware problem. Else there will be no notifications at all. But most time everything is alright. And only sometimes the device behaves as if SMS never arrive.