Instant Messaging Notifications Suddenly Stopped working on iPhone 7 Plus

Hi,

I have an iPhone 7 Plus on iOS 15.7.3. All of sudden last week, my instant messaging apps such as WhatsApp and Telegram stopped pushing notifications out; causing me to missed a whole lot of messages my colleagues have been sending to me. The only way I will receive the messages is if I keep the app open front and centre. One I close the app or lock the screen, the messages stopped coming in.

For the life of me, I can't seem to find out what's wrong and what I did (or didn't do). Here are the steps I have already taken:

1. The phone HAS internet connection; both on cellular and WIFI. I know because I can still surf and receive mail.
2. The phone is NOT on Battery Saver Mode.
3. All Notifications for the above mentioned apps are turned on under Settings, and have turned them off and on for good measure.
4. I have checked and Background App Refresh for these apps are ON and I have turned them off and on for good measure.
5. I have restarted the phone multiple times and done a soft reset.
6. I have reset Network Settings multiple times.

HELP!