On the mobile app:

Select a chat from the list and tap on > 'info' icon on the app bar (on top)

Tap on > 'Switch' to turn the notifications 'ON' or 'OFF'

If a specific chat has notifications muted, it will show the following slashed bell icon in the room list

On the web version:

Select a chat by clicking on a chat from the chats list and click on > 'gear' icon

Click on > 'Switch' to turn the notifications 'ON' or 'OFF'

If a specific chat has notifications muted on the web, it will show the following slashed bell icon in the room list on that specific chat box

Note: By default, the notifications are allowed i.e. 'ON'. The user has to turn off or mute the notifications manually.