1.i wrote a script and put under /etc/ppp/ip-up.d, but the system doesn't run the script when the ppp0 is up.
2.i did chmod a+x

3.the script:
var1=`ifconfig ppp0 | grep 'addr:' | sed 's/.*addr:\([^ ]\{2,3\}\).*/\1/'`
if [ "$var1" != 218 ]; then
    ifdown wan && ifup wan
else
    WANIP=$(ifconfig ppp0 | grep addr | awk '{print $2}' | sed 's/addr://')
        iptables -t nat -A prerouting_rule -d $WANIP -p tcp -j DNAT --to 192.168.1.100
        iptables -A forwarding_rule -p tcp -d 192.168.1.100 -j ACCEPT
        iptables -t nat -A postrouting_rule -s 192.168.0.0/12 -p tcp -d 192.168.1.100 -j MASQUERADE
fi

3.1.the purpose of the script: Keep reconnect until get a wan IP start with 218 (218.x.x.x), if the IP is start with 218 then it'll create a loopback firewall rule.

use /etc/hotplug.d/iface and check for ACTION=ifup and INTERFACE=wan
that also works with dhcp and static ...

nbd, thank u very much~~ The router is now able to get WAN IP which start with 218, and create loopback firewall rule.

/etc/hotplug.d/iface/10-218
if [ "$ACTION" = "ifup" ] && [ "$INTERFACE" = "wan" ]; then
var1=`ifconfig ppp0 | grep 'addr:' | sed 's/.*addr:\([^ ]\{2,3\}\).*/\1/'`
if [ "$var1" != 218 ]; then
    ifdown wan && ifup wan
else
    WANIP=$(ifconfig ppp0 | grep addr | awk '{print $2}' | sed 's/addr://')
        iptables -t nat -A prerouting_rule -d $WANIP -p tcp --dport 8080 -j DNAT --to 192.168.1.100
iptables -A forwarding_rule -p tcp --dport 8080 -d 192.168.1.100 -j ACCEPT
iptables -t nat -A postrouting_rule -s 192.168.1.0/8 -p tcp --dport 8080 -d 192.168.1.100 -j MASQUERADE
fi
fi