Some routers display the admin login detail along with the password on the case. Sometimes "admin" gets you in using the password "password".
To get in, press your Windows key and R together then in th eRun bx that pops up type
cmd /k ipconfig /all
then press Enter.
Among the information that shows up in the black Command form is the Default Gateway. It might look like 192.168.0.1. That is your gateway to the Internet and it's also the address you need to type into the URL bar of your browser as though it was a website address, to get to the router's login page.
Quite often, bandwidth slowing down is a matter of geography or settings within Windows. What have you tried so far?