Port forwarding to an Ethernet connection

Logging on to the admin portal with an XFi gateway XB7 (the white box). In earlier versions you could explicitly set up the port forward. Now it sends you the the app. The app only lists target computers on the Wi-Fi. I have a old computer (a DEC VAX, from the era before Wi-Fi) on an Ethernet port on the gateway. How can I get into the details of the admin portal (or get the app fixed) so I can set up the port forwarding as I want?  Before you ask me all kinds of newbie questions, I can Telnet (port 23) using the Putty client from a computer on the WiFi to the old computer on the Ethernet port just fine. The issue seems to be the app we're forced to from the admin portal won't recognize computers on the Ethernet port. If it just had an expert mode so the admin portal could be given a 10.0.0.X address of the old computer on the Ethernet port, and a port number to forward, there would be no problem.