I've got 3 Decos directly connected via ethernet to my moder/router, and the setup is in access point mode. It worked fine for a while, but recently the two secondary Decos started ignoring the wired backhaul and are connecting to the Main deco via wifi. The main reason I have these is that we're in an ancient house with extremely thick walls and the signal across them is very weak. Thus the signal that reaches my office is no longer sufficient to support video calls, for example. Any ideas where the error may lie? The cables seem fine, I've tested them directly. I've rebooted everything, and shifted into router mode before returning to AP mode. 

I can't do images here but the topology as wired is simple: each Deco is connected with a (fairly long) ethernet cable to the modem. 

As executed: Main Deco to Modem via ethernet; Deco 2 to Main Deco via wifi; Deco 3 to Deco 2 via wifi. 

Thanks in advance.