Just to be clear I am referring to an Internet Service Provider. I am assuming we both know what that is.
In my previous experience I could not retrieve my router Default gateway username/password. I contacted my isp (comcast) though their online tech support and asked for them to reset it. They were able to reset it, however it may be possible that other ISPs do not offer that service.
Thou has broadband.
ISP gave you the username and password you used to connect to the ISP itself, this is a case in which you plug the network cable straight into the PC, I am required to input a username and password (of course I have it on remember).
If you buy a router, you must input a separate username and password for the router access, and then the broadband connection above. There is however nothing stopping you from putting them both in. Your "ISP giving you the username / password" is from the actual ISP connection, which you configured to be the same on the router. You can have "admin / blabla123 " for router and then "qwnfd3klnflk / ewfgin32jr02" for ISP. This is a situation only in broadband as far as I know.
That, or your ISP gave you a router which was intentionally pre-configured like that.