@UkDavid 

When you have ISP router you need to keep, running Deco mesh in AP mode is the right answer.

First of all, if you ever have issues with Internet link, it is much easier to convince ISP Tech support troubleshoot it when it is their router. Replace their router by something else and any issue reported will be initially brushed off as "third party networking gear - not our problem."

Deco mesh will work better in AP mode: Main Deco CPU will not be taxed by content filtering, bandwidth throttling, antivirus, etc. You are getting practically all CPU in Deco unit for just handling WiFi traffic and passing it through, no overhead.

Ethernet backhaul becomes much more flexible with Deco mesh in AP mode: you don't need to pass all traffic through Main Deco.

If you could wire Satellite Deco directly to router, or to router through switches - you should do that, bypassing Main Deco. That will offload Main Deco even more, because all Internet traffic going through that wired Satellite Deco will pass directly through the router and won't go through the Main Deco.

There is also undocumented Deco mesh resiliency feature for when you have more than one Deco wired to the router in AP mode: if Main Deco fails, wired Satellite Deco will take over as a temporary Main Deco. LED on Deco units may switch to Red and you will have degraded WiFi coverage if you lost single Deco unit (Main Deco), but the rest of Deco mesh will be operational providing WiFi coverage. 

Satellite Deco with WiFi backhaul should reconnect to wired Satellite Deco in that setup, as long as distance between them permits it.

--------------

After initial Main Deco setup and configuration, switch Deco mesh to run in AP mode and then add remaining Satellite Deco units to your Deco mesh.

  @Alexandre. Thank you for such a brilliantly thorough and clear answer! I really appreciate that. Thank you!