Yes, you need the IP address.
disconnect from wireless if your computer is connected to it.
(You may resume this case afterward at
Then connect your computer with a Ethernet cable, to a common port on the router (not the Internet port. One of the numbered 1-4 or 1-5 ports will do.)
Then open a Command Prompt window.
Then enter this command
and press RETURN on your keyboard, once. Get the 'Default Gateway' address, which should be four numbers separated by . (dots.)
Then enter that Default Gateway address in a browser, and you should get the prompt or popup for login to the router.
for the username. Do you have the password ***** the router? if not, try the default passwords, either 'blank' i.e. no password, no characters, nothing, or else
which are common default passwords for the admin account that allows Setup of your router.