It depends on your model. Some dont use a username, some models use root, and some use admin, and one model uses 1234. There are only two passwords listed, 1234, and admin.
If these dont work, then you would need to set the router back to all defaults possibly causing an outage. To reset it, press and hold the reset button for 30 seconds.
You have to be wired. You have to be in same network as router.