I have a Uniti Atom and I absolutely love it!!
A problem I think I have came to light in the last few dayswhen I tried to connect to my new Roon Nucleus, when it seems that nothing plugged into the USB sockets, front or back show any activity.
I have had no joy connecting my Nucleus to either the Atom or my Chird Poly, so I think the Nucleus has problems which I’ve told them about, but it won’t help my case if the Atom is not receiving anything through the USB.
The USB sockets on the Atom are for connecting USB storage - sticks, drives etc…
I’m fairly sure your Nucleus is meant to be connected to your Network - looking at a picture of the connectors there’s an ethernet port for such purpose.
If you want to directly connect the Nucleus to the Atom (without using the Ethernet connection method mentioned above) you will need a USB To SPDIF converter, of which there are many available.
Some are also designed to reduce the electrical noise associated with USB 2.0 connections.