Just to be clear, if it was ever connected to Wi-Fi in the past, then there is a previous connection, and if you know the SSID/passphrase, then you can use that. (The previous connection trick is putting this info into the new router, not the roku.)
Until the Roku gets connected, it doesn’t have an IP address. It doesn’t even know the local private address range that your new router might be using.
Banned but back. Because why not?