Is the Nucleus setup to use a DHCP assigned IP address or a fixed?
If fixed, I’m wondering there is now an IP address conflict (if you used the the same IP for Grimm).
I would suggest using a network scanning app like fing to see if it can identify the Nucleus.
Alternatively, have a look at your router’s web config page to see if the Nucleus is listed.
If that still does not help, the next step would be to connect a HDMI screen (and USB keyboard) to the Nucleus so you can view it’s console page.
Hope that helps.
PS as you’re selling the Nucleus, to save the purchaser a headache make sure it’s configured to use DHCP so it will obtain an IP address from their network when it connected.