find the extension you want from the website for example: https://extensions.gnome.org/extension/771/proxy-switcher/
the number behind the name of the extension inside the url is the id of this extension. note it down.
run this command with the id of your extension
gnome-shell-extension-installer
restart the gnome shell with this command. do not panic if you see a black screen. just wait.
killall -3 gnome-shell
open extension manager and enable your extension.
walall