D6400 - locked out of router

Hey, first post so apologies if I do not know the etiquate!   So today I manually updated the firmware of my router and now I cannot log back into the router through an internet browser.

I can give the serial number and answer the security questions.  The screen then shows that I have successfully recovered the admin password.  It tells me that the Router Admin Name is : admin and gives a password (not one I have set up ever).   I try to log in with these credentials but it doesn't work.

Is anyone able to help me get in to the router another way or advise how to get the password changed?     I would really like to avoid having to restore to factory settings as when I got the router it took forever to get it set up with a lot of troubleshooting!

Thanks in advance to anyone who can help this technophobe and all the better if its simple jargon!  

Thanks in advance