As Liquorice said, the easiest way is to switch the TP-Link to Access Point mode, (if it has one), then skip to *. If it can’t do that then it is still doable but this is where it gets more complicated.
Two things to remember. You can’t have two routers in a network, so you are not using the TP-Link’s router function. Nothing should be plugged into its WAN port or DSL port. Secondly, two DHCPs in a network will cause problems. (Not strictly true, but the ISP’s kit is usually not up to it and you would need to know how to configure them to work together anyway).
Basically, to make this work you have to switch everything in the TP-Link off, except for the wireless function.
What you also need to do is give the TP-Link a static address inside your network but also outside the range the BT hub is issuing. I seem to recall the BT smart hub is already set up to allow addresses 1 to 63 to be used as static addresses. Go into the TP-Link and set it to use 192.168.1.20 (They both need the same subnet mask, if it asks for it. Usually this is 255.255.255.0 on ISP’s equipment. If necessary, check what the BT hub is using).
*You will also need to turn off the wireless function on the BT hub. (The two Wi-Fi’s may interfere with each other if you don’t).
You then connect the BT hub to your master socket, (or ONT if full fibre), and the TP-Link from one of its ethernet ports to one of the BT hubs ethernet ports.
