You can’t use that IP for ROCK. That is for the sMS-200. Your set-up should look like this:
NUC (Intel) ethernet 1 port:
ip: 5.1.1.1
netmask: 255.255.255.0
gateway: nothing
dns: nothing
NUC (USB Ethernet adapter) ethernet 2 port:
DHCP (you can set-it up as static IP but use the IP within your subnet.)
sMS-200 IP:
ip: 5.1.1.2
netmask: 255.255.255.0
gateway: nothing
dns: nothing
ROCK will provide the IP of your LAN network that was set in DHCP. For example: 192.168.0.x or 10.10.10.x. This is the IP you will access from your Roon remote (browser). IP’s 5.1.1.1 and 5.1.1.2 cannot be accessed from your browser because they are now in a different subnet.