There will be a couple problems with this setup.
One is that the the Ethernet FeatherWing only supports DHCP addressing. It does not support static IP addressing, so it won’t work on an isolated network that doesn’t have DHCP.
The other problem is that as soon as the Ethernet has link and an IP address, Device OS will attempt to use it for the Internet/cloud connection, even if the network is isolated and can’t connect to the Internet. It will never fall back to cellular once the Ethernet interface is up.
With a sufficient amount of effort and modification of Device OS it may be possible to work around these issues, but it’s not a trivial change.