Can't log in to Plusnet on newly installed TG582n Plusnet router

[#1]

I configured the Gateway with my Plusnet username and password and that's solved the problem! It's not a step that's mentioned in Plusnet's setup instructions, so thanks very much for suggesting it.

[#2]

Good news.  I'd get in touch with support anyway and get the router moved to your account to prevent any configuration issues in the future or follow this guide to disable TR069 on your router.

[#3]

Is the TG582n router PlusNet branded?
If the answer is yes, there is every possibility that the router is registered with a different (defunct?) account.  Therefore the 'normal' auto configuration process might well have been unable to function correctly.
As recommended by Pwatson it is essential that you contact support to get the router serial number allocated to your account.
What level of firmware does it have?