At this time there is no way to set a static IP address on the Ethernet interface. It should be possible to do that either by code or USB, but I don’t believe it’s possible at this time.
If you have control over your DHCP server, you can assign a static IP by MAC address over DHCP. That’s how I set them and it eliminates the need to manually configure static IP addresses on each device.
One important thing is that the Ethernet interface must be enabled. If you used the mobile app setup and checked the Use Ethernet FeatherWing checkbox, this flag will be set. It’s stored in configuration flash.
It’s also possible to enable Ethernet from code. The reason it’s not auto-detected is that in order to probe for the Ethernet interface, a few GPIO and the SPI need to be used, which could adversely affect non-Ethernet peripherals connected to those pins.
There are some methods in the Ethernet class.
But mainly you just need to use TCP and UDP like you would on any other device.