@user255, I also use an Eero Pro 6 and ARC works like a charm. This is my setup:
The Eero sits behind my modem. In the modem settings page I have added a NAT/PAT rule for ARC: set port 55000 open to the Eero IP address (TCP only). Depending on the make of modem, the naming can be different.
Now your Eero has access to this port number.
UPnP is not selected (also safer).
In the Eero Network Settings → Reservations & port forwarding, I have added a Reservation for the Roon Server IP address (your Nucleus). Fill in the address of your Nucleus, set the Port of Port Range to 55000 and select the TCP protocol.
Toggle the Port Forward on.
The UPnP setting is Off.
That should do the trick. Happy ARCing.