RE: For PPPoE users (the WAN side of the router) who receive an IP Address, then cannot access either the web portal, or several websites./usr/sbin/iptables -t mangle -A POSTROUTING -p tcp --tcp-flags SYN,RST SYN -m tcpmss --mss 1421:65535 -j TCPMSS --clamp-mss-to-pmtuWorked! Whoo hoo!

Googled 'Chillispot MTU' and eventually found this post. My users on the public side could not connect to several websites, including Hotmail.
I have confirmed this does fix the bug in DD-WRT's version of Chillispot where the MTU problem in the Chillispot TUN/Virtual adapter, causes some websites to 'hang' forever on 'Waiting for xxxxx.xxx'
This will end up in the DD-WRT Wiki.
It seemed like an MTU problem. Earlier, adjusting the 'physical' LAN port MTU under DD-WRT, made no difference.
It makes sense the TUN adapter has it's own MTU.