Which is why BT supply you with a fibre modem if you have a Mk4 router - like they did for me when I upgraded to fibre.someone-else wrote:
A BT router Mk4 does not work directly on fibre optic broad band, where as a BT router Mk5 does.
It seems that the modem part of the MK4 is the problem, and one way round it is to have an external modem. (The other is to get a MK5)
So even though the router is advertised as suitable for broadband from a variety of ISP's it is NOT suitable if you have fibre optic broadband (.............which comes down the same phone line as the other sort of broadband)
If you look carefully at the ports the Mk4 has ADSL and WAN whereas the Mk5 has ADSL and Infinity
You never stated in your original post that you had fibre broadband so how would people have guessed your problem??