If it wasn't you wouldn't have got an IPv6 address in the first place. I suspect the problem lies with the Billion, I had exactly the same with a TP Link 9980 with beta firmware. TP link never did produce firmware that worked with BT's implementation of IPv6 for the 9980. If you can borrow a HH5 or later with latest firmware, you can verify if that works or not whether it is the Billion at fault.