Snapshot replica to remote NAS/Snapshot vault?

[v.customer]

Hello Forum,
I look for information regarding replicating/backup one or more iSCSI lun from one QnapNas to another. I searched the Web and the forum to find reliable and recent information from other users or from qnap, but was not successful. Maybe it is working that simple and good that nobody posted about this topic? Maybe if did not search at the correct places? Or it does not work at all, and I simply are misinterpreting the qnap marketing blurb?
Therefore I made an account in the forum and here goes my question:
How to replicate a iSCSI Lun across a WAN link for backup purposes with QTS 4.3.3.0188?
I have a qnap TS-853A with 8x4TB Disks raid-6. A part of the space is used for plain files, another part used by iSCSI Volumes (blocklevel, thin-provisioned) 4TB size. These Volumes are mounted by a Windows Server 2012R2. The windows built in Backup software is used to backup the Machine to the iSCSI Volume. To have an off-site copy of the backup until now we made a copy (weekly) of the currently used backup volume to a local USB disk using the Lun Backup function. But this involves too much manual efforts, so, I thought why not buy a second Qnap and use the shapshot replica function and the vault on the remote Qnap NAS.
I made a small test by creating a 1GB Lun, writing a file onto it. created a replica job to the NAS itself. Compared the size of the copied data with what I put on the drive. Copied another file to the drive, and did rerun the replica job, noted the amount of data. Only the actual data size was copied, during the second run, only the size of the newly added file was copied. Good. It looks like an incremental Backup is performend - if this would not be the case, a backup across a slow WAN link would be impossible.
Deleted the LUN and target, went to the Snapshot Vault. There is a button "restore" but it does not function properly. However, you can choose "clone". I was successful cloning the last saved snapshot, mounted the restored Lun on the server, and my two files were still present. In case of a disaster I would carry the NAS from the remote location to the Office, then clone the LUN back, setup the Windows Server, mount the LUN, restore the backup.
But I cannot try this with a connection through a WAN link since I would not like to buy another NAS for $1800 and then only have the hope it will work. The daily changed data is about 15GB. With a 10Mbit WAN link (IPSec Tunnel), it should be possible make a daily backup to the remote NAS within 5hrs. But I dont know about how much overhead is added by the qnap software which does the replication.

Does this really work that easy? Has anyone experience if this is the correct way to backup an iSCSI LUN to a second NAS connected through a slow WAN link?

Thank you.

[v.customer]

I can now answer a part of my question myself:
I still dont know the overhead the qnap replication will have, but, I found out I can do a test replication to the NAS itself over the lan. Even, If the Microsoft Server backup told me, only 15GB of data was added by the incremental backup, the replication to the snapshot vault replicated 250GB (the complete backup is 1.8TB). So, even if the overhead would be 0, a replication over the wan is impossible unless I had a 100MBit Fiber upstream Link.

I now will try a backup software with built in WAN replication. If this works better I could still use a qnap NAS, as source or destination, but, using the qnap delivered replication make no sense if the remote NAS is on a slow link.