Hmmm this is frustrating, could you describe what is happening in more detail? I am confused why it works when you have the Smarthub ahead of the pfsense router. I am not familiar with configuring the smart hub as a bridge, so where is the pppoe authentication done, in the smarthub or pfsense?
Are you getting a v4 address?
Is the v6 interface up?
Are you getting a link-local v6 address?
Dumb question, but I am going to ask it anyway, are you using your bt business pppoe username/password?
cheers
F