@StevenL You can buy the Ethernet module separately; it is available to replace a defective Ethernet module on an existing card, but can also be used to convert a WiFi card to Ethernet.
You will need to desolder the WiFi module, and replace it with the Ethernet module; the underlying Duet 2 board is common to both, unlike Duet 3 Mini 5+. You'll need some two 6x1 low profile header pins to solder into the board for the Ethernet module to plug into, and two plastic 5mm high standoffs. I think these are supplied with the replacement module.
There are a couple of threads on doing it, this is the best I think (from 2017):
https://forum.duet3d.com/topic/2890/converting-a-duet-wifi-to-a-duet-ethernet
Ian