This sounds nit-picking, but isn't.
Your description of your wiring isn't clear to me. You actually say the BT point in the house upstairs, with a 4-metre extension cable to the router. You may mean a 4-metre extension cable running upstairs from the BT point to where you have the router.
I assume by the BT point you mean the Master socket. Or do you mean an extension socket and the Master is somewhere else completely?
The Openreach engineer will fit a socket and supply and fit a VDSL modem plugged into it, near to the Master or on a dedicated extension up to 30 metres from it. They seem to be willing to reposition the Master if asked.
From the rest of your post I guess you are with Zen, and they are supplying a (cable) router? That connects to an ethernet port on the Openreach VDSL modem.
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