I'm a little confused as to whether the BT Broadband account username and password has any significance. Let me explain. I have just upgraded the Broadband plan on one site and BT sent me a new Business Hub 5. I didn't need it as I have installed a TPLink 9980 VDSL router there, so I decided to use it at another location where I have BT Infinity but with the separate HomeHub 3 and Openreach modem.
However, when I came to install it I couldn't remember my BT account password (I knew the username from the HomeHub 3). So instead I just tried the username and password from the other site and it worked fine. What's the score there ? I assume I am not using the account from the original site as that would be a major loophole in BT's accounting. Therefore it would seem that the username and password are redundant and it purely goes on knowing which physical line it is connected to, in which case why is there any need for login credentials at all ?

One way to test the theory would be to enter a totally made up username and password and see if that connects.

Without a valid username and password you wouldn't make it past the radius server.

After a conversation with BT support it turns out that password for the username bthomehub@btbroadband.com is blank (i.e. no characters). Tried this and it works.

Been like that for years, when I was on BT ADSL i could put anything in and it would work, but Default was normally BThomehub@ what ever BT broadband called themselves at the time.
Next door neighbours is the same.