If you're helping a friend, relative or colleague to troubleshoot a problem with their broadband connection, the quickest option is often to perform a factory reset on the device and then set it up again from scratch. The only problem, of course, is that you then need to know the default username and password to which the device will have been reset, in order to log back in and configure it correctly.
You can generally find such things by searching the internet for a phrase such as "default password" followed by the model number of the device in question. Alternatively, there are some useful ready-made lists that you can print out and save for reference. One such list is at http://www.phenoelit-us.org/dpl/dpl.html for example.
Such lists also highlight a useful security reminder: always change the default passwords that come with any device you own. It should be the first thing you do when you start the setup and configuration process.