Hmm . . .
With the USG disconnected from everything (other than its power supply), link its LAN port to a computer with a standard Ethernet patch cable. Open a web browser and enter
https://setup.ubnt.com into the address bar.
You should then see
either a login page (if the USG has been adopted by a UniFi Controller
or the USG Setup window. If the former, just enter the credentials you have configured for the USG.
Which ever of the above two options was true you now need to get to the USG Setup window. Left-click on
Edit Configuration and configure the WAN port for PPPoE. You should then be presented with two boxes for your ISP/CP's login credentials. Enter
bthomehub@btbroadband.com and
BT. Save the configuration.
Disconnect the Ethernet patch cable and plug all the equipment back together. Power-cycle the USG and wait.