Great question — answering as the UphillTech team:
If I understand correctly, you basically want that NUC to function as a switch between your eth1 and wlan0? If so, you can create a network bridge with iproute2 ( ip link add type bridge ... ) or with bridge-utils ( brctl addbr ... ) and add the two interfaces. It's not required to assign an IP addresses on those interfaces. In that case, your eth0 will be the only interface with an IP address and will just work as if it was the only active network interface.
Full answer by Tom Cannaerts: https://unix.stackexchange.com/a/732413 (CC BY-SA 4.0).
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