"............and said I would be able to use the TP-LINK router using WAN ports. Could anyone specify the correct setup to use?"
The BT engineer was correct.
Using an Ethernet cable, connect the Openreach modem to the LAN/WAN port of the TP-Link, login to it, click on "Quick Setup", choose "Wireless Router Mode" then select "PPoE". The username is [email protected]. There's no need for a password, but if the router insists on one then try password or BT.