I bought a Roon Nucleus three days ago and despite every effort, I cannot link it to my two Roon Ready devices, a Naim Uniti Atom and Chord Poly/Mojo. The Atom is connected via USB 3 cable and this and the Nucleus are connected to router via Ethernet cables.
Also, the Nucleus said it had downloaded my 6000 tracks but on checking the installed SSD it has only 7.4 Mb of used space.
Something is wrong I think!
There has not been a hint of music via Roon link to either DAC device though the display would make you think that contact had been made.
Just out of interest have you updated the firmware on your Mojo/Poly? It needs to be at least version 2.0 to be a Roon Ready device. I recently purchased one and had to update the firmware before Roon would see it.
You also need to use the GoFigure app and connect to your Mojo/Poly to your wireless network and turn on Roon Ready mode: