Hello,
I experimened and found the way to create a on/off script for opening/closing transmission port. When is on, transmission can be acessed from the WAN (and Internet). It seems that only port 9091 is needed for the web client and .Net client. I'm using transmission version 1.75
Code:
#!/bin/sh

case $1 in
    "on")  iptables -I INPUT -p tcp --syn --dport 9091 -j ACCEPT;;
    "off") iptables -D INPUT -p tcp --syn --dport 9091 -j ACCEPT;;
    *) echo "Please enter on/off as argument !!! !!! ";;
esac
iptables -L INPUT