@DeclanMC 

When Satellite Deco M5 loses Ethernet backhaul, even briefly, it will switch to WiFi backhaul. During the switch Satellite Deco will not have connection to Deco mesh. Satellite Deco will force all devices connected to it reconnect elsewhere. These devices will keep staying connected elsewhere for some time, even after Satellite Deco regained WiFi or Ethernet backhaul.

Looking at your diagram, if all three Satellite Deco lose all clients simultaneously, the suspect becomes switch they are connected to and Ethernet cable which connects both switches. Also, it is possible the bigger swtich which is connected to Main Deco could be experiencing ussues, but these should have been easier to notice.

I suggest you start by checking power supply to the switch with three Satellite Deco connected to it: is it properly secured, perhaps it is loose where it is plugged to power outlet or to the switch.

Then, check (and maybe replace) Ethernet cable between two switches. Check first, I once had Ethernet cable getting loose and that costed me hours of troubleshooting until I have found the culprit, with very simple fix: I unplugged cable and plugged it back tightly.

Thanks for the advice. 

What I might do is factory reset everything. And slow add back in each node day after day. Perhaps one of them or the cable going to one of the nodes is damaged and causing issues with the switch. 

I did double check all ethernet jacks and make sure they're solid. Same for the DC cables going into the switches.

I also looked closely to make sure the colour coding for the pairs within the ethernet cables was right (WhiteOrange-Orange, WhiteGreen-Blue, WhiteBlue-Green, WhiteBrown-Brown). 

Will update here anyway after a few days and see what I can find. 

Ok, some progress I think. 

I was able to find that by plugging in Deco 4 (in Diagram) via ethernet to the Switch, was causing Deco 2 and 3 to start their red lights flashing. When I plugged it out, they came back to health. 

I did have a spare ethernet cable in a different room and connected Deco 4 to this instead, and then connected this to the switch. So far so good.... 

So it seems the cable that was originally supplying Deco 4 is faulty, or may need to be re-terminated?

Is there an easy way to test Ethernet cables? I do have this kit, but not sure what to actually do with it to test the cable? 

Found a Youtube Video showing a easy guide on how to use these.

Turns out the cable is fine (the legacy one connecting Deco 4). 

The only thing I can thing of now is that I had another ethernet cable going from Deco4 into my laptop via USB-C dongle. Is it possible the Dongle could be faulty, and this is messing everything up? To be honest I'm not sure exactly whe the network started playing up, but the USB-C dongle is probably the newest thing to my home network setup...

I switched out the USB-C dongle now for a standard USB-A > Ethernet I had lying around. Everything seems "ok".... for now....

Well I'm back to square one today / over the last 24 hours. 

I've lost ethernet backhaul again suddenly. I know if I go factory resetting again and connecting one-by-one it will probably work. 

I think I might try the Amazon Eeros and see if I have better luck with them over the course of a week or two. 

So I took away all switches and am now just using one 5-port TP-Link switch to supply the two satellite nodes. 

So far so good (again) but as usual this takes a few days to see if it is stable. 

Ok, I think I've had the longest period of success now. 

Previously, my first "Unmanaged switch" was a D-Link and the second one was a TP-Link. 

I changed over the D-Link one now to a TP-Link LS1008G. 

That makes it two TP-Link unmanaged switches now. Everything is working smoothly it seems. All satellite nodes show as connected via Ethernet in the map on the app. 

Some things I noticed through this journey: 

• I expect it was a firmware update in July that broke my setup. Which is weird; I never had any issues previously with the D-Link switch setup etc. 
• Looks like TP-Link switches are mandatory now for me. 
• The Deco app is awful for giving you an up-to-date picture of your network/setup. I would pull out ethernet cables or connect Ethernet cables to various nodes and it would sometimes take the app 3-5mins to show them as either connected via Wifi or Ethernet. I would close the app, kill it, log back in etc but it looks like the updates sent back to the TP-Link servers and subsequently the Deco app are delayed.

Final update (I hope). 

I've now reverted to using my ISP-provided router and connected this to the Fibre ONT box. 

I then factory reset all Decos, and re-connected one by one; but instead now just assigned the main Deco a Dynamic IP and the put it in "AP" mode.
I disabled Wifi on the ISP's router. 

I connected all other satellite Decos via Wifi first, then ethernet afterwards.
I did this on Friday, and so far no drops of connection (as of Monday). All Decos are now showing Ethernet Backhaul also. 

My thinking is with the prior setup, some kind of loop issue was occurring. Even using TP-Link switches didn't resolve this. 

I'm getting full speeds now as provided by my ISP, but annoyingly I have their router now plugged in when for years I never needed it. Some kind of firmware update may have broken my setup for some odd reason.