To set up working guest access behind a FIOS or Comcast router requires setting the WRT 1900 Internet port to use a fixed IP address. The FIOS or Comcast router is directly Internet connected and sets up internal network as typically a 192.168.n.0 with router being 192.168.n.1 network. DHCP scope could be 192.168.n.50 to 150. WRT 1900 fixed Internet IP address is 192.168.n.3. WRT 1900 wireless network becomes a 10.n.n.n network. Wireless devices access Internet and 192.168.n.n devices because WRT 1900 provides NAT. NAS storage port works but 192.168.n.n devices must use FTP to access it. FTP access must be enabled, and I enabled FTP port triggering and FTP port mapping. Wireless network 10.n.n.n devices have direct access to the Internet and the 192.168.n.n network. Server access required using the server IP address (or a hosts file address setting). WRT 1900 setup must be done on wireless network or using wired device attached to WRT 1900 switched ports. Wireless web cams are accessed with web browsers. WRT 1900 uses port mapping to map port 88 to a web cam IP address. Guest access now works for wireless devices with the login screen required to access the Internet. This is a network within a network setup that works with some minor inconveniences (e.g. FTP NAS access) and some fancy (but not difficult) foot work to configure the router.
I hope this is helpful.
Pete the nerd
"Your friend on a technically challenged planet (c)"