If you have disabled network notifications by error you can try the following commands (user shell doesn't matter):
gsettings set org.gnome.nm-applet disable-connected-notifications false
gsettings set org.gnome.nm-applet disable-disconnected-notifications false
gsettings set org.gnome.nm-applet disable-vpn-notifications false
Cinnamon still has no native graphical applet for this. Meh.