@Sulde Guest network is available since version 1.21. If your version is earlier than 1.20, you need to manually upgrade to v1.20 first. See instructions here: pcWRT firmware v1.20 for TORONTO-N is available for download.
After you upgraded to v1.20, go to the System Settings page, click on “Check for Updates”. You might need to reload the System Settings page (or clean the browser cache) in order for this to work properly.