Linksys EA6400 only shows one xbox in device list

I'd take a look at the configuration of your switch(es)? Do the switches show up as wired devices on the router? It's possible that the switch is NATed and only sending it's traffic listed as coming from itself to the router.

Not sure of your setup for what could be done easily for troubleshooting, but I'd see what happens if you swap the switches and see if the OTHER Xbox starts showing up, but not the first? Maybe see what happens when the not listed Xbox is plugged directly
to the router or both going to the same switch.

I do not have the option to bypass the Linksys system.

I only have one Xbox one console connected. No other gaming systems.

I have removed the port forwarding rules and UPnP is enabled.

I have rebooted the Linksys device.

It still says moderate.

Do you have the option to bypass the Linksys and connect your Xbox directly into the Ubee device?

Do have more than one Xbox console connected to the same router/NAT Internet connection?

Remove the port forwarding rule that you configured and make sure UPnP is ticked enable and reboot the Linksys.

I did that, though--like I said, I registered it using the serial number and now it's in my list of devices, but I still can't do anything with it from there like install to it. It also doesn't make sense to me that the console having a previous owner
would be the issue; the only reason there's an Xbox One in my list of devices is that my friend has one and I logged in one time to play Minecraft. I don't own that console, obviously, and it's not registered to me, but signing in once was enough to make their
console automatically show up in my list of devices.

GTA V is a different beast , Rockstar servers can be tricky for anybody at times ...are both of you just using wireless to hook up you xbox ones to the Linksys router ? When you search for wireless networks , does the arris and the Linksys both show up?