As others have mentioned its important to highlight the difference between a sync (basically a replica of the source) vs a true backup which is historical data.

As far as tools goes, if the device is running OMV you might want to start by looking at the options within OMV itself to achieve this. A quick google hinted at a backup plugin that some people seem to be using.

If you're going to be replicating to a remote NAS over the Internet, try to use a site-to-site VPN for this and do not expose file sharing services to the internet (for example by port forwarding). Its not safe to do so these days.

The questions you need to ask first are:

1. What exactly needs to be backed up? Some of it? All of it?
2. How much space does the data I need backed up consume? Do I have enough to fit this plus some headroom for retention?
3. How many backups do I want to retain? And for how long? (For example you might keep 2 weeks of daily backups, 3 months of weekly backups, 1 year of monthly backups)
4. How feasible is it to run a test restore? How often am I going to do so? (I can't emphasise test restores enough - your backups are useless if they aren't restorable)
5. Do you need/want to encrypt the data at rest?
6. Does the internet bandwidth between the two locations allow for you to send all the data for a full backup in a reasonable amount of time or are you best to manually seed the data across somehow?

Once you know that you will be able to determine:

1. What tool suits your needs
2. How you will configure the tool
3. How to set up the interconnects between sites
4. How to set up the destination NAS

I hope I haven't overwhelmed, discouraged or confused you more and feel free to ask as many questions as you need. Protecting your data isn't fun but it is important and its a good choice you're making to look into it