There would be a use for the ethernet port in that you'd place the plug/extender in your cabin where the WiFi doesn't reach and have it connect to the internet via the ethernet cable you noted in your post - this would give you an additional stable WiFi access point to connect the smart plug...
TRY THIS
Hopefully that works - remember to buy through the affiliate link to help @Bozza out. Bozza think there's an issue where Amazon links are appearing as unloadable media rather than links.
I think this may be as simple a fix as buying something like this (link with NSC affiliate) and plugging the ethernet cable into the LAN port. This device will then act as a wireless access point that you can connect to via WiFi (including a smart plug).