At the very least, you need to have a DHCP server running somewhere. I use
dhcpd32 under Windows. You'll have to have already setup the Roku with access to the Internet or I think it will choke on the "Connecting to Internet" stage and it will never work, but I'm not sure. Once you've got the Roku all happy, setup the DHCP server to give the same local IP address to the Roku that it was using when it had access to the Internet. You shouldn't have to go through the network setup again and you probably don't want to as it will just choke when it tries to connect to the Internet. I just tried all this with my old N1000 running v3.1 of the firmware and it seems to work. I assume the v5.x firmware will act the same. I know in the past I've also had a Roku connecting to my laptop via a wireless connection with the laptop connected to a router via a wired connection, but in that case the Roku was able to get to the Internet. I'm sure I could have accomplished the laptop to Roku wireless connection without an Internet connection, but I never tried.
It may be easier to use that old router you have lying around, but again I think the main trick is to have setup the Roku completely with an Internet connection and then match the IP address it was getting when you move it to the router without an Internet connection.
-JT
Roku Community Streaming Expert
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