Forgotten username and password for my RT-AC66U Asus router. Does anyone know what to do in this case?

the default settings for this router

On your web browser, manually key in the wireless router’s default IP address: 192.168.1.1

default user name (admin) and password (admin)

If you need to reset the router hold down the reset button at the back for a few seconds

If its a router supplied by your provider, it might have the details on a sticker or a card, if you yourself set this then simply do a 30 30 30 reset. Hold down reset switch for 30 seconds keep holing it and unplug it, for another 30 then plug it back in (keep holding it) for another 30 then turn off and on, it should be factory reset.

Is it written on the back / bottom?

Hard reset