It seem like that device can't be put into router/bridge mode. I would suggest calling talktalk and asking them if they can provide a device that supports router/bridge mode - this will enable you to set up everything, like DHCP, port forwarding, etc... on the Pi.
Also, unless you are completely happy with the RPI wifi, I suggest buying a USB3 ethernet dongle (for the wan), and having a seperate device acting as a dumb wifi AP. Here is an excellent guide on how to set this all up (HOWTO: OpenWrt on the Raspberry Pi 4).
Good luck!