Yes, the notification setting can be restored via
gsettings. The reason you didn't find the setting is because the it is not stored by Xubuntu notification daemon
Reset network notification
The easiest way is to reset the network notification to default setting from command line. The following one-line command will restore notification when the Internet connection is established.
In the Terminal, type the following command and press Enter key.
gsettings reset org.gnome.nm-applet disable-connected-notifications
The effect is immediate, so restart is unnecessary.
To see the notification, click on the indicator icon for networking in the Xfce panel, then select/deselect
Enable Networking option. The notification will show up again as usual.
Extra: Other network notifications
In case you accidentally pressed the Don't show this message again button for other network notifications, you can list them by running the following command.
gsettings list-keys org.gnome.nm-applet
The output will show list of keys as below.
Look for relevant ones that have
notifications keyword. Then run the reset command with the key you wish to reset.
gsettings reset org.gnome.nm-applet [KEY]
Alternatively, you can choose to enable notification manually by running the set command with
false option. Since the key names are named as
false option is correct.
gsettings set org.gnome.nm-applet [KEY] false
Both methods have been tested working in Xubuntu 14.04.