In Gargoyle (firmware based on openwrt) to access your adsl modem in bridge mode you need to:
1-Turn on dhcp server on the modem
2-Set openwrt router lan ip to 192.168.2.x (not 192.168.1.x)
3-Edit /etc/ppp/ip-up.d/modemaccess.sh to set your modem ip:
vi /etc/ppp/ip-up.d/modemaccess.sh
set ROUTER_IP=192.168.1.254 "ip address of thomson tg585v7"
press Esc on your keyboard
then type :w and press enter to save
you are able to access your modem(via router) after doing the above steps