RND 2000 V2 Ready NAS DUO Wrong IP address

I'm assuming you set a fixed IP address on the NAS with your old router.  That's never a good idea, and you have found one reason.  Once you get this sorted out, reserve an IP address for the NAS in the router instead.

Your computer and the NAS have to be on the same sub-net.  While you cannot "expand" the router to include your old subnet, you should be abe to change it to that.  Another option is the direct connection to the computer, but you have to manaully set the IP address of your computer to be in the same sub-net as the NAS.  To do that in linux, use ifconfig to find which port you are using (probably eth0) then ifconfig  eth0  10.X.X.X netmask  255.255.255.0 (assuming it is etho, and then using an address with the same first three digits as the NAS and the last one different).

As for the OS re-install, it sounds like you've had your NAS a while and would genuinely know if it's a real V2.  But just to be sure, a real V2 says so on the front, not on a sticker on the bottom.  If it doesn't say "V2" on the front, it's a V1, and the boot menu instructions are different.