Usually ReadyNAS Replicate is what we use on that situation but I think ReadyNAS NV/NV+ is not compatible with ReadyNAS Replicate. So, I guess you can try doing what StephenB suggested.
I suggest ZeroTier. It's a VPN, but co-exists with your standard IP connection rather than replacing it. The two NASes can be on the same ZeroTier network while on completely different LANs anywhere in the world (some firewalls excepted). Do a forum search on ZeroTier and you'll find my post on it and a link to the package.
Once you have ZeroTier set up, you just specify the ZeroTier IP instead of the local one in a standard backup job, and it just works. You can put the one that will be remote local initially for a faster initial backup, leaving the ZeroTier configuration the same, and the traffic will go over your local network instead of via the internet. I've toally abandoned ReadyCloud in favor of ZeroTier. Besides doing NAS-to-NAS communication which ReadyCloud does not do, I use the same credentials and drive mappings at home and when away.
OOps. I somehow wasn't thinking about that by the time I got to the end of the thread. Definately not an option for direct use. OS 5.x may have a chance, though nobody has built a package for it. 4.x is just too old.
RSync over SSH is probably the best option unless you want to bucket brigade it through a constantly-on PC (or even Raspberry Pi) on the same net as the NV+.
