The SX Installer does not connect to a server for you to download updates or games. It simply is an installer of the files you have in your sd card, or whatever directory your switch is connected to.
So what you need to do is to add a server as a directory in the file browser of sx installer/tinfoil.
Search 'jits' online, do what needs to be done, and it will be added as a server sx installer/tinfoil can connect to.
Then it will work as you expected it to work in the first place. (Another alternative is installing hbgshop)