DDWRT advance setting: Block all traffic except HTTP HTTPS and FTP

add the following commands in DDWRT setting.
(Administration-->Commands-->paste commands-->Save Firewall.

iptables -I FORWARD 1 -p tcp -m multiport --dports 21,80,143,443,465,587,993,995,7001,7002 -j ACCEPT

iptables -I FORWARD 2 -m state --state ESTABLISHED,RELATED -j ACCEPT

iptables -I FORWARD 3 -j DROP

or 

iptables -I FORWARD 1 -p tcp -m multiport --dports 21,80,123,143,443,465,587,993,995,7001,7002,8443,8605,10001,10002,22345 -j ACCEPT

iptables -I FORWARD 1 -p udp -m multiport --dports 53 -j ACCEPT

iptables -I FORWARD 3 -m state --state ESTABLISHED,RELATED -j ACCEPT

iptables -I FORWARD 4 -j DROP

Notes:
IMAP: 143,587
thinkorswim: 7001,7002