I'm pretty sure that I have bandmin server running (and yeah, i've just changed username + password of bandmin service); however, when I type in the URL to bandmin page, it just shows a 404 error page.
I tried both:
http://serverip/bandwidth
http://server.com/bandwidth

anyone know what's wrong?

try
http://ip/bandwidth/

That's what I tried.

Is there an official support website for bandmin or anyone knows how to run bandmin in command lines? I can't seem to find its manual in man bandmin

Notice the trail difference between yours and mine. If that does not work too, check your web server configuration if an alias for bandwidth is defined correctly.