Stephen,
The router should be set to use the WAN port with PPPoE. However, make sure the incoming connection is set as BT (VDSL). Because that router is an ADSL or VDSL model, if it is not set to VDSL it will not connect to FTTP properly. The fact it is so slow would make me think that is your issue.
Other than the username and password for PPPoE, you shouldn't need any other settings. DNS can be left to be set automatically, although you could try using Cloudflare to see if that helps. 1.1.1.1 and 1.0.0.1 are their DNS servers.
I hope you get more help than I've ever got from these useless idiots. Do yourself a favour and switch to a decent ISP.