@Jim_Hong, have you checked whether the option “Roon Ready” is (still) set in the administration interface of the Stream Box 2S Ultra, under “Settings → Sources”)?
If you have made a firmware update, it is possible that the settings have been reset.
Seems stupid but make sure you didn’t press the USB/PC button and it’s still on NET. I say this because it happened to me when I had the S2 Ultra and pressed that button by accident then the S2 Ultra disappeared from Roon Audio list.
After a power interruption the S2 may need to be “reset” by pressing the button on the far left of the front of the unit. Wait until the two blue LEDs on the front are both lit and then the S2 should appear in Roon.
How long do you have to hold that button for? I held it down for what felt like a full minute and nothing happened until I RELEASED the button. Then the far left LED started blinking orange (which I believe means it rebooted). About a minute or two after that, the middle LED (NET) turned blue.
But the S2 still does not appear in the list of devices.
i suggest that you try a full reboot: unplug the power, wait a minute, plug the power back in, briefly press the far left button and wait until both blue LEDs are lit. These little boxes can be quite finicky at times.