@Marcel.... after investigation it turned out to be one single old cat 5 cable somewhere hidden in the entire network.

@Alexandre. If i do that i loose the benefit of the deco router features and the m9 will simply turn into an independent access point.

LS My internet model provides 300Mb/s throughput When i do a wired speedtest directly after the main deco i get only 75Mb throughput Same on any deco wired or wifi connect device downstream Test...

I have currently setup the following network, and this works fine for now ISP modem (bridge mode) ->Ethernet -> M9 (Master, router) -> Ethernet -> unmanaged switch -> 6x Ethernet (all via own cable)...