OK - everything you have is gigabit - to do what I'm about to suggest may cause a temporary loss of connectivity, you should be able to regain connectivity by reconnecting to the router, worst case scenario, you'll need to do a reset to defaults using the pin switch on the rear.
Try manually configuring the NAS on all the network options - start with gigabit, full duplex, then half duplex, if you can't maintain a connection at gigabit, try 100mbit, again, both half & full, and eventually drop to 10 mbit.
By the way - what I asked was where did you get the cables - not what sort of cables - the reason is this. A CAT5 cable will only meet CAT5 spec if properly terminated, likewise a CAT6 cable will only meet CAT6 spec if properly terminated - there are very specific techniques required to meet those specs if you're terminating the cables yourself.