@David-TP 

Thank you for the prompt reply. I bought the X50 Deco's to have a simple, reliable, mesh network.

I would really like to resolve this issue as when the Deco's work, they work really well. I would like to connect Deco as such:

Fibre->PON->(ethernet) -> X50 primary (ethernet) -> 10 port dumb TP-Link Ethernet switch (ethernet) -> 6 wired ethernet connections one with a second 10 port unmanged switch, another wired backhaul to my second X50 -> ethernet to second office PC connected through the X50.

Additionally I have about 30 Home automation devices/TV/Amp/Phones/cameras etc connected via WiFi. Mixed WiFi versions from 2.4 to wifi6 The Decos have a wired backhaul but it seems unreliable through the dumb ethernet switch. It switches to WiFi backhaul on occasion I run a lot of automated jobs over the network and it seems the devices can not see each other at certain times. It is annoying when devices just disappear off the Deco network. I do not have this issue on my old network. I suspect it may be related to the Deco doing network optimisation, or on old firmware trying to re-connect to PON, but this is only my theory. It seems DHCP/DNS also fail at this time.

I would prefer to connect the Deco in router mode directly to the PON and eliminate all other routers, but this has proved remarkably unstable.

So I connected my original ZYXEL EMG3525-T50B (Disabled the WiFi) to the PON as router then ethernet to one Deco in Access Point mode. The rest of the ethernet connections plugged direct into the ZYXEL. This too proved unreliable, wifi devices just could not connect to internet at times. Currently I have reverted to my ZYXEL as router and Wifi (Supplied by my ISP with their custom firmware) and my TP-Link Archer C20 as wifi access network extender for the devices out of reach of primary wifi. Decos in cupboard...

I do also have an old TP-Link TL-WA855RE as network extender over WiFi for some security cameras but I only include this for info as I have not changed that part of the network at all.

We do live in South Africa and electrical power is unreliable and power cycles are a part of daily life, not sure if this adds to the issue.

Currently the Deco is flashing blue and I cannot connect to it at all, as App will not let me log in when connected to the Deco Wifi i cannot get any info out of the Deco.

I have given up - I cannot use these Deco's for mesh WiFi. I will try again when there is a better/more reliable  firmware. My old router at least works, but no Mesh.