cannot “insert” from url

I have some hundreds images uploaded via ftp in a folder on my server /public_html/pics organized in subfolders like: /public_html/pics/botany /public_html/pics/geology, etc, while my WP is in: /public_html/wp.

The idea was to find something that could list all the files inside the folder “pics” and display their thumbnail, automatically updating the list with files added or deleted. But I could not find any way (with a plugin, because I cannot code) to do what seems to me a so simple task… Why???

For many reasons I cannot move neither the images folders nor the wp home, so I resigned to make galleries adding manually the URL of each image; unfortunately, all the gallery plugins that I have tried don’t have this feature, taking in account only picture from the Media Library. I resigned more and thought that I had to: 1) put the link of images inside the ML with “External Media”; 2) make Galleries with another plugin

To see if the first stage of my workaround works, I activated the URL plugin of “External Media” but I don’t find the option: “Only allow insert to remote files” while there is for the other external sources like Dropbox, Instagram etc.

Can someone help me suggesting how to untangle? Thanks!