Install the network-tools addon (from libreELEC)
This addon will install the rsync in the following folder:
$ which rsync
/storage/.kodi/addons/virtual.network-tools/bin/rsync
Now, if you try to sync the files, you will get this error (probably):
$ rsync -av my_local_folder root@X.X.X.X/storage/videos
sh: rsync: not found
rsync: connection unexpectedly closed (0 bytes received so far) [sender]
rsync error: error in rsync protocol data stream (code 12) at io.c(228) [sender=v3.2.3]
You need to specify the path for the rsync on your destination host
$ rsync -av --rsync-path=/storage/.kodi/addons/virtual.network-tools/bin/rsync my_local_folder root@X.X.X.X:/storage/videos
Done, you will be able to sync your files.