You’d have thought that a google search for “manually add plugin to WordPress” would turn up lots of articles on how to do this, but, er, no. They all seem to tell you to log in to the site and do this or that on the GUI. That’s not doing it manually – it’s using the GUI. If the GUI doesn’t work, then you need to do it manually. Here’s how:
The method is actually simple if you remember it. You download the plugin from the site (e.g. https://wordpress.org/plugins/) and you’ll end up with a DOS/Windows .ZIP file. Unpack this any way you wish and you’ll get a directory with some file in it. As a sanity check, one of these files will have the same name as the directory, but ending in .php.
Take this whole directory and copy it to /wp-content/plugins. That’s the directory – not the files in the directory.
That’s it. You’re done. It’ll appear in the plugins dashboard.