Pre-requisites: sys-usb VM
If your router needs a locally attached cable in port 1 or something similar, and you only have a USB RJ45 adapter, then you need to have a net-vm with the adaptor attached and a VM who uses this net-VM as network:
[Max@dom0 ~]$ qvm-usb
sys-usb:3-8 5986:0535 Generic_Lenovo_EasyCamera_200901010001
sys-usb:3-2 0b95:7720 ASIX_Elec._Corp._AX88x72A_10FD4D
sys-usb:3-1 046d:c52f Logitech_USB_Receiver
sys-usb:3-7 04f3:016f ELAN_Touchscreen
sys-usb:3-6 2047:0855 Invensense_Lenovo_Yoga_31F3806F24001100
sys-usb:3-4 8087:07dc 8087_07dc
[Max@dom0 ~]$ The above command shows the USB network adapter (in Bold). If you are uncertain, unplug device and try again…. the difference is key :))
Attach the device to the net-VM which will use it with the following command:
[Max@dom0 ~]$ qvm-usb -a lokal-belkin sys-usb:3-2
[Max@dom0 ~]$ 
The above 3rd listing is the network card. Also in dom0, you have 2 network interfaces up in the right corner:
The above will setup as default and try to get ipv4/ipv6 dhcp.
The above shows successful address with dhcp.
The network is now working, and the x is gone. Happy times.