Don't hear notification sounds on iPhone 12 Pro Max

I am a new customer of apple company and i just bought an iphone 12 pro max 3 months back only and from beginning I don’t hear the notification sounds nor do they come up on the notification bar in this new iphone. I had to open the manually some apps after that only i receive emails and Whatsapp messages. Even i don't get calls on my Whatsapp messenger and if i open the app it shows directly that i have a miss calls. All these problems i shared with apple support team and they told me to do this do that and everyday i had to waste my time for that like one hour two hour for that but still that problem is not resolved. now i requested them to please replace my device but they are saying that they are not authorized to do that. i am requesting that anyone is there who can help me out to solve my problem? or apple company just like to cheat people with giving faulty products? even i have my case number if any responsible person is there who can solve this out they can go through with that. . If apple's new product has this kind of a problem than how anyone will trust this company?

[Edited by Moderator]