After a little searching I believe I have found a solution that should work for you. Yandex disk is accessible via WebDAV and Ubuntu has tools available to mount this type of remote filesytem. Once this is done you would have a folder in your filesystem you could copy files to normally but those files would actually go to your Yandex disk (not a sync, the folder would actually exist on Yandex Disk).

• The WebDAV details for a Yandex disk can be found here.

• A set of instructions for mounting a WebDAV filesystem on Ubuntu can be found here.

Check this out
https://rclone.org/

You can add it to CRON and sync any folder.