I have flashed my Netgear R6250 router to the DD-WRT Kong mod dated 20170514. The process appeared to complete successfully.
When the flash completed a screen popped up requesting login credentials change, which I did. Now when I http to the url I receive a system information page but when I click on any of the setup or administration links a login box appears and the credentials which I originally entered are refused.
How can I regain access to the router?
That happened to me on both of my routers. Have you tried a factory reset by pressing and holding the rest button for 30 seconds? When the router comes back up you should be able to set your username and password.