Adding Sky Q main box with wired ethernet upsets local area network and Plex

[NodesOfYesod]

My dad recently upgraded his old sky hd box to sky q and sky router

Its a fairly large house with 3x TP Link Deco Access Points, model PX50 with integrated powerline plugs rated at 1.5Gbps

His Win10 PC runs a Plex Server and it all works flawlessly. Plex Server supplies several Amazon Firesticks dotted around the house running Plex Client. Media plays instantly and never drops out.

Cue the introduction of the Sky Q main box and router.

I'm not interested in running Sky Q's WIFI Mesh. Its simply inadequate and prefer a modern 3rd party solution especially given the size of the house and the existing Plex system, hence his TP-Link Deco which work flawlessly.

I disabled WIFI on the Sky router and on the Sky Q main box (he has no use for a Sky Q mini box)

I plugged in the Sky Q main box via ethernet and immediately, Plex struggles to provide any type of streaming, giving a revolving circle icon in the middle of the screen which lasts forever.

Can anyone explain what's going on? It seems the Sky Q main box is saturating the LAN with traffic, unncessarily.

[NodesOfYesod]

Note above

Fig 1.0 - with Sky Q box wired into LAN, causing issues

Fig 2.0 - no Sky Q box, everything working perfectly

Access Points working in access point mode - no routing or DHCP