Ady, surely that IP address is the home page of your own router, where you can configure the router settings
MyBT is your home page on BT's servers, where you can access your account details, emails, payment methods etc.
Bear, I'm no longer with BT, having moved to Plusnet when I retired to Buxton.
As mentioned above, BT did change their email provider away from Yahoo Mail, which I understand had lead to some issues for some customers.
Your email account(s) should be supplied free with your BT Broadband account for as long as you remain a customer.
They only charge for it if you leave BT, as I did to Plusnet, and wish to retain the use of your BT email accounts when using a different provider for your broadband.
I've just been having a quick play on your behalf and managed to re-activate my old MyBT account;
If you try and log on using your username (which I assume you know), it gives you the option of having a 4 digit pin sent to your registered email address, which it what appears to have happened to you.
I don't know where to put it either, as I didn't use that re-set option.
If you have another attempt at resetting your password, but rather than accepting the 4 digit PIN to email option, claim (presumably falsely, as you say you've got a PIN) that you can't access your email.
The system will then ask you one or more security questions, and give you the option to re-set your password on-line, by entering a new one into a box, and confirming it in a second box.
Worked for me
Good luck
Bill