Lubuntu 21.04 - I frequently suspend my machine, but find the Network Manager notification pop-ups annoying. In Preferences, LQXT Settings, Desktop Notifications,Advanced Settings, there is an entry box for “Ignore these applications” but I am unsure what would go in there for the NetWork Manager?
To toggle notifications on connecting and disconnecting to networks right click on the nm-tray tray icon and check or uncheck the checkbox for Enable notifications. See the manual for more information.