|Is the BT fibre terminated to an ONT? Most likely this is a modem/router.|
What is your TP router status showing on the WAN side? something like 192.168.100.1?
I had a similar problem when I changed to fiber internet connection. The ONT installed is WUAHEI modem/router
I forwarded all ports (DMZ) on the ONT to the WAN IP address of the D-link router.
The ONT now acts as a pass-through for all traffic.
Another solution is to set the ONT as bridge and do the internet termination on the external router (DHCP, Static IP, PPPoE, etc.)