@SEGA RPG FAN Okay, I have a NVG589 router that I've configured into bridge mode so I can use an Orbi as my router. So, I guess that's the device getting the public IP since it's acting as my modem. There's not a specific setting for DMZ but there is one for Passthrough. Is that the same thing?
On that page one of the options is "Default Server." The description is "Allows the device to forward all externally initiated IP traffic (TCP and UDP protocols only) to a default host on the LAN. You might need this if you cannot anticipate what port number or packet protocol an in-bound application might use. For example, some network games select arbitrary port numbers when a connection is opened. Or you might want all unsolicited traffic to go to a specific LAN host.
Here's a screenshot of the IP Passthrough page: https://i.imgur.com/jYaQTJ0.jpg
That sounds like what I need. Is it? edit: Googling tells me I should be using passthrough for my router but it won't let me type anything into the Default Server Internal Address field. Guh.
Also, there seems to be a place to port forward. Here's a screenshot: https://i.imgur.com/Pv5Ph8W.jpg
If I'm going to do that instead, what do I put in the Base Host Port field?