This is a bit of a public service note, combined with a brag about solving a problem(!)
I bought a kit of three Deco M5 units because I wanted to have seamless wifi service through my house without having to manually log in to the nearest wifi source (I had two old Hub 5's as access points)
Deco could apparently use ethernet backhaul (doesn't have to rely on wifi to communicate between units) and I already had ethernet everywhere so it was the best option for me. It would also happily operate in AP mode, so I wouldn't need to lose my SmartHub as main router (it has all the port-forwarding set up).
Anyway, Decos are set up using your phone and bluetooth, via their app. The first one was fine, the second a bit flaky and the third one crashed the system really hard - I had to factory-reset the SmartHub to get it working again. Wifi speeds even with two Decos operating were strangely slow and flaky. After several hours struggling I was ready to sell them on eBay.
But! I did a load of googling and came across a reply on a TP-Link discussion thread that looked relevant; the manufacturers engineers reported having seen issues with certain ethernet switches not passing a data protocol that the Decos use. These were all D-Link switches. I looked at my network, with four D-Link switches, and realised I might have a possible solution. I replaced all the switches with Netgear ones and tried again with the Decos - this time all went like a charm.
Now the system is stable and solid as a rock, and my mobile gadgets all have three bars of wifi wherever I go in the house, with no apparent lag in switching. I am very happy with it.
But for anyone else with D-Link switches - beware. It seems that they don't pass through data using the IEEE 1905.1 protocol, which is relatively new and was designed to balance data flows across different transmission modes (wifi, ethernet and anything else).