Username and Password TG582n FTTC

[#1]

I have upgraded the router to the latest firmware, but had some issues with the interface, so I decided to downgrade the firmware, which went without issue. When restarting the router after the firmware downgrade, I can no longer get into the web GUI. All combinations of username and password do not work! (Admin, admin & Administrator with no password; Admin, admin & Administrator with serial number)
Help Anyone!

[#2]

Did you use a plusnet firmware or one from a different site?
If it's plusnet firmware
Username = admin
Password = Serial number printed on base of router ( starts with CP)
Both are case sensitive.
If it's not plusnet firmware, try:
Username = SuperUser
Password = serial number.

[#3]

It might also be worth doing a factory reset.