I'm on Plusnet and have a Thomson router. I've no idea if it's the same model as yours but on mine the routers configuration page is accessed via
http://192.168.1.254/.
You have to set up some port-forwarding rules which on the Thomson is done via the Toolbox using the "Game & Application Sharing". As best as I can remember this is the method...
1) Select "Create a new game or application"
2) Give it a name you can remember and set it to "Manual Entry of Port Maps".
3) Set the "trigger" to TCP, "port range" to 8080-8080 and "translate to" to 80.
4) Now select "Assign a game or application to a local network device"
5) Under "Game or Application" in the drop-down box select the name you entered for step 2)
6) Under "Device" select "<User defined...>".
7) Enter the IP address of the TiVo in the new box that just appeared.

Click ADD.
That should be it. No need to reboot or save settings. You can access your TiVo from your 3G phone but you have append ":8080" to get the cirrect port address, for example:
http://123.132.213.231:8080It's a real good idea to also edit the tivoweb.cfg file on the TiVo to add a UserName and Password, otherwise every passing wannabe hacker will be in there messing up yout TiVo.
EDIT: For

read 8) ... stupid bbcodes.