don't think it can be work as a LAN port even in AP mode.

try to add another wireless powerline extender with three ethernet ports. then, you can add another two ethernet ports.

https://www.tp-link.com/en/products/list-18.html?filterby=4895&page=1

Thank you for your reply. Actually, I already have an UTP cable running from the central hub to the room, where I need to connect 4 Ethernet devices and have an WiFi AP.

I solved the issue by adding an 8-port Gigabit Ethernet switch to connect all 4 Ethernet devices + the Archer C1200 configured in AP mode. But the hardware is piling up.

With the good-old DD-WRTed Linksys WRT-54G, I was able to reconfigure WAN port to join LAN bridge, effectively making it a 5-port switch+AP and it covered all the needs for that room. Until I wanted to upgrade to Gigabit Ethernet and AC WiFi.

Basically, the AP option into a router firmware menu has to put the WAN port to the LAN.
Unfortunately, I'm not sure that works with all the routers like with DD-WRT...