Where is "System Settings > Network" ?

I attached what I mean.

Try opening System Settings and then scrolling down.

All the 'System' settings are further down, underneath the 'Personal' settings.

'Network Connections' is there, which sounds like what you are after.

I also noticed that this is notably missing from Toutatis. There might have been an important package that was not included in the Trisquel repositories. Any suggestions?

I found the cause of the problem. Installing the gnome-control-center-data package found in the Ubuntu repositories makes System Settings-Network available, but makes others like System Settings-Backup unavailable. It probably has something to do with the categorization of the settings, as the Ubuntu version has some settings listed under "Hardware", and the Trisquel version categorizes settings under "Personal" and "System" only.