System will not switch to wired mode

I have four fixed speakers and a Roam. The wi-fi in the house is a mesh network (BT WholeHome). In wireless mode I suffer frequent audio cut-outs with the message “your wi-fi network is unstable”.

To fix this, I switched to wired mode by plugging ethernet from my router into one speaker. For a while this worked much better, but today I noticed that all my speakers except the one directly connected to the router are back in wireless mode (appear as WM: 1 in ‘about my system’). I have tried rebooting all the speakers, the router, and removing and then plugging back in the ethernet connection but the speakers will not switch to wired mode. The speaker connected via ethernet shows as WM: 0. 

(I realise that the Roam will have to stay in wireless mode)