@EstebanZarza 

Hello, it depends on how did you set it up as an access point.

Below is the instruction to configure it as an AP.

https://www.tp-link.com/support/download/archer-c6/#video

If you changed the operation mode to access point on the web UI, the WAN port should be connected to the main router by wired cable;

While if you set it up as the AP manually ( turn off DHCP server, change LAN IP), you need connect one of the LAN port to the main router.

May it help. 

@EstebanZarza Right, that I know, but what actually changes in the behaviour of the WAN port compared to one of the LAN ports?

@EstebanZarza 

Hello, with WAN port used, the data will be handled by the CPU, that makes some features still alive even though the router works in AP mode, like smart DHCP. 

While with LAN port used, no CPU involved in. No corresponding features available either.

Good day. 

@Kevin_Z That's interesting, I was wondering the same in my C6. Using it in AP mode (plus I also set a static IP and set DHCP to Off instead of Auto because I don't really understand what Auto means...). I find no difference at all between using the WAN or any LAN port besides having an extra LAN port available for wired devices.

Is the WAN port like a fifth LAN port when in AP mode or what is the difference at all? For wireless devices it's clearly none, but for wired ones?

In fact, @EstebanZarza , I prefer using the LAN port when working as AP so wired devices are just in the 4-port Gigabit switch (I don't really know if using the WAN port effectively makes the C6 work more to inspect packets of wired devices, but just in case I prefer it just acts as a regular switch for the aforementioned wired devices)