@Ohgivemeabreak Lets start again.
What physical connections have you made, what ports are connected where?
That only applies if you are connecting it to a modem.
I an sure there was a setup wizard to auto detect the connection type.
@Keith_Beddoe The OP hasn't specified how he is connecting the device. We don't know if he is connecting to a BT Hub or directly to the ONT.
Just see https://www.tp-link.com/uk/support/faq/2464/
Connect the first unit to an Ethernet port on the home hub, the Deco will see that as a modem and allow Internet access.
You then change it to access mode, it then reboots, and that is it.
You can then add extra nodes via the app.
It cannot get any simpler.
First post "I'm fibre to cabinet, then good old copper wire to property."
Missed that
If you simply follow the instructions, it automatically gets an IP (LAN) address from the home hub to start with. That allows an Internet connection to register the device against your TP Link ID.
You then simply change the mode from the advanced menu.
If you think you have done it wrong, then simply factory reset it, and start again and let it detect the connection type.