@mlj 

You ask if this is going to work:

Yes, it will.

Still, there is a better way to do it, with Deco mesh running in Access Point mode. You can keep Main Deco connected to the router and have Satellite Decos connected to the switch which connects to the router. 

This setup is better, because you will not be passing unnecessarily all your home Internet traffic (wired and wireless) through Main Deco.

  @Alexandre. Thanks for the quick reply ! I already tried the second way, but it didn't work.

So Main Deco connected via cable to router, router connected to switch.

When I connected the satellite Decos to the switch via cable the whole network hung. It was only when I removed the LAN cables from the satellites that things started working again.

The above was after a full reset of all Decos on site, and setting them up via wifi first.

It looked to me like there was some kind of a network loop?

  @mlj 

There could be, indeed, network loop because after you plug Ethernet cable to Satellite Deco for the brief period of time it has both WiFi and wired link to your home network.

Dumb switches don't care and everything works, because Satellite Deco starts using Ethernet link only, but if you happened to have managed switches - these might. If your switch has network loop protection, it is recommended to disable it, at least until Satellite Decos settle down.

Another possibility: if you happened to have switch that is not compatible with Deco Ethernet backhaul, it won't work. D-Link switches are known to have issues with Deco. 

More on that topic is here: General questions about Ethernet Backhaul feature on your Deco

I am running my Deco mesh with Deco units connected through two unmanaged switches, from TRENDnet and TP-Link, and lucky me: they just work with Deco. Never had issues, I've got lucky with my networking gear.

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To prove it is the switch and to confirm it is not the router or anything else. Bring two Satellite Deco to the router and connect them directly to Archer VR2800 Ethernet ports. Power them on one after another. If all your three Deco units (Main and two Satellites) are happy in that configuration and Satellites report Ethernet link, you have narrowed down the problem to the switch.

  @Alexandre. Good idea, thanks. The switch is a smart managed Dlink I think, but will check next time I'm there

Thanks again

  @mlj Sorry, quick follow up question. So in AP mode, the main Deco has an Ethernet connection direct to the router. Does that put the Deco into Ethernet backhaul mode ? Or does it also need a second Ethernet connection to the switch behind the router that the other Decos connect to? (if that makes sense)

So current setup is:

Decos in AP mode

One port of router (Archer) connected via Ethernet to DLink switch.

One port of router connected via Ethernet to one of the Main Deco's Ethernet ports

Remaining Deco units currently Wifi only but will be hardwired to the switch (I'm aware of the DLink compatibility issues so this switch will probably be replaced)

  @mlj 

With Deco mesh running in Access Point mode, you can connect Main Deco to either router (Archer) or to the switch. That will put Main Deco in Ethernet backhaul mode, assuing switch is compatible.

For example, this will work:

  @Alexandre. Just for info - I haven't been able to use the switch (a Dlink DGS-1100-26MP) due to the IEEE 1905.1 packet forwarding issue.

They have, however, sent me a firmware update this morning which is supposed to fix this.

Once I've applied this and tested it I'll update this post.....may be cheaper solution than replacing the switch.!