You can manually FTP upload representatives into an accompanying pwg_representative/ directory, for example if there is a video upload/2020/05/02/filename.mp4 then the representative would be upload/2020/05/02/pwg_representative/filename.jpg
Copy the index.html from the parent directory into the pwg_representative/ directory to prevent directory listing regardless of how your server's indexes option is set.

Ah darn, I forgot, it needs an entry in the database as well, table piwigo_images field representative_ext the filename extension e.g. jpg (which would happen if you synced from an FTP folder or had it generated from the video which you can't).

[extension by plg] Photo Update is said to be able to update the representative, I never tried.

Looking at plugins/photo_update/admin.php source code it also enters width and height of the representative to the database, but I don't know if that is necessary to have it displayed at all.

As it does that if ('representative' == $file_to_update) my guess is it offers to upload a representative as well if you upload/update the main file, I didn't investigate deeply.

Shrug, I'd rather kick the hosting provider who doesn't want to install ffmpeg.