WDR3500 reset button with OpenWrt

I'm reading the situation here is a forgotten root password. OpenWrt typically runs with only one user, "root". Attempting to log in with a different user name will fail. Note that when you run ssh on a PC, it defaults to trying to log in to the remote machine with your PC user name. You need to specify with "root@" or "-l root" to log in to the other machine as root.

That is easy to fix with failsafe mode and a PC with a SSH client. The SSH client comes standard with Ubuntu. The 3500 has nothing tricky about entering failsafe mode.

Start with the router power off. Be ready to press the WPS/Reset button but do not press it yet. Turn the router power on and wait until the "gear" LED (second one from the left, next to the power LED) starts flashing. This will be about 10 seconds after you turned the power on. Immediately when you see it flash, press and release the WPS/Reset button several times rapidly. If you were successful, the "gear" LED will now be flashing very quickly. The router is now running in failsafe mode.

The failsafe mode will stay running until you change it, so you can set the router down and connect one of the LAN ports to your PC. Set the PC to a static IP of 192.168.1.10 and a netmask of 255.255.255.0. Then:

ssh root @192.168.1.1

You will be prompted to either accept the SSH key, or if you have SSHd to another device at the same address before, be told that the key has changed and that seems really bad, and you need to run a command to forget the old key, then run ssh again. Once you get past that you should see a page of instructions from the router and the OpenWrt command prompt.
If it is just a forgotten password you should do mount_root then passwd then reboot. If you aren't sure what is wrong you could do firstboot, that will require you to reconfigure everything like from a new install.