World of warcraft doesn't need any port forwarding. You are connecting to WOW, wow isn't connecting to you.
But the thomson's are know for their intrusion detection being as pain in the butt and causing problems. So I could see that causing your problems when WOW tries to connect to the online servers.
Adding another router to the mix wont help. You either need to disable the intrusion detection in the thomson or get a new modem/router all together.
You will need to have telnet installed on your system. If you need it installed just follow this.
Click the Start button Picture of the Start button, click Control Panel, click Programs, and then click Turn Windows features on or off. Administrator permission required If you are prompted for an administrator password or confirmation, type the password or provide confirmation.
In the Windows Features dialog box, select the Telnet Client check box.
Using telnet you can disable the intrusion detection like this.
Open a command window,
telnet dsldevice.lan
username: Administrator
password: <your password set for this user>
ids
config state=disabled
saveall
exit
Shane