The easiest way to do it is to do a factory reset -- that way you know what the IP address, user name and password are. Connect a wired cable from your laptop/PC to the DIR-825. Then configure the router as an access point as shown
here. Be sure that you set the IP address of one that is NOT in the DHCP range of your primary router.
Set your WIFI password, etc. Connect a cable between a LAN port on the DIR-825 and your main router.
You should have an access point.