WAN Port - DD-WRT Wiki
https://wiki.dd-wrt.com/wiki/index.php/WAN_PortOther firmware builds of DD-WRT have the same or very similar steps. Open up the Web Interface to your DD-WRT Device Set WAN 'Connection Type' to Disabled. Down the Basic Setup page, the option WAN Port appears. Check the box 'Assign WAN Port to Switch'. Save/Reboot The WAN port is now the same as the other LAN ports.