A while ago I had big problems with Demon broadband because they "upgraded" the service and made it incompatible with my router. After a bit of back-and-forth Demon kindly replaced the router with a newer one, a "Thomson TG585 v7". It works fine.
While trying to get our radio station back online and streaming, I discovered something dodgy about the router setup, so if you happen to have one of these routers then do this check described below, to make sure your router's admin page isn't exposed to the world. I have to thank the very helpful people on the portforward.com forums who spotted the issue (thread here, with more details).
(1) Connect to the router's admin interface using telnet. On my Mac I do this by launching Terminal and typing telnet 192.168.254.254 (then giving the username and password when prompted).
(2) Type config dump (and press return) and a massive massive screed of text will appear, listing all the config settings for the device.
(3) In that text, look for a subsection labelled [ servmgr.ini ] (for me it was near the bottom). Check to see if these lines are in that bit:
ifadd name=HTTP group=wan ifadd name=TELNET group=wan
The important thing here is "wan". "lan" is OK, it means you can have local access to the admin, but "wan" is dodgy because it means you're providing an opportunity for the world to access your router.
(4) If you do have those lines then you can fix the situation by running the following commands (the final one will reboot your router):
service system ifdelete name HTTP group wan service system ifdelete name TELNET group wan saveall system reboot
Voila. After rebooting you may wish to go through the steps again to check that the config settings have been changed.