Guest Zone is for allowing guest connections to the Internet on the WAN side of the router and does not allow any connections to the LAN side network and private areas on the LAN side. Its a one connection point for users to connected to either using a password or unsecured with no password. The normal AP connection has connections to both WAN of course and access the LAN side on the private network.
If the Guest Zone option is not listed in the routers settings, then the 605 is not supporting of it. I looked at an emulator of the 605 and don't see Guest Zone listed. Routers like the DIR-655, 645 and other DIR series routers have the Guest Zone option.
This one only seems to have the normal AP zone which if you did unsecured, would allow anyone connecting access to your private network.