@Sunshine 

network topology is as follows 

modem connects to other brand router which connects to X50 deco in access point mode, from this main deco one ethernet connection is made  to a X50 satellite deco and one Ethernet connection is made to a TP link switch, (from the satellite X50 deco ethernet connections are made to two separate P9 decos , from one of these P9 decos there is a connection to another P9 deco and from this P9 Deco there is a connection to another P9 deco)  from the switch that is connected by Ethernet to the main X50 deco there is individual Ethernet connections made to two X50 decos and to one P9 deco. No other connections on the network. 

this configuration has worked perfectly with full stable Ethernet backhaul across both deco types on the network for greater than 9 months until 3 -4 days ago when now only the x50 decos are Ethernet backhaul connected.
 

Inoticed yesterday that my P9 decos need a firmware update ( not sure if this is linked to issue) however as of yet I have been unable to update software which unfortunately often seems to be the case but eventually it works 

hope this helps