WhatsApp won't work on my M31 mobile. It works only I open it otherwise it's not getting any messages. So I wanted to run my WhatsApp on background. How to make it?

I'm using M31 mobile. It's working nice but am getting WhatsApp messages while opening WhatsApp app only. Otherwise it not get or show messages.Simply WhatsApp works only when I open the app.So am losing important information

Samsung M31 or many phones of M series has no app lock feature built in.Its difficult to use 3rd party app locks.I want an app lock feature in the future update.People Do comment below and share your thoughts.