the majority of routers that have a built in "access point mode" actually require you to use the Internet/WAN port for uplink when they are places in access point mode, as they will request DHCP out of that interface.
Some of them have all of their interfaces bridged to LAN and it wont matter.
If your router does not have a dedicated access point mode in the firmware, then you are correct, you would need to use a LAN port.