Right now I use my deco x55's in rounter mode. The issue with this configuration with an ethernet backhaul:

ISP modem/router ---> Deco #1 ----> network switch ----> end devices / Deco's #2 and #3.

The issue I have is that any time I lose internet connection (which occasionally happens due to where I live) the entire network dies. I have since learned it's an issue that has to do with the WAN port autosensing no longer working when the internet is out and not properly distributing IP's to devices on the network. When my network dies though sections of my nearly completely local smart home stop working as well as i'm unable to access my local media server nor can I access my local security camera system. Wifi devices are completely unable to connect unless they are within range of Deco #1. In another thread I saw that V1 of this product has a firmware update that allows manual setting of the WAN port but nothing for V 1.2 which is what I have. Hopefully someone can help it becomes a serious issue any time the internet is out.