@jamieman5 

TP-Link positions single M5 as addition to existent Deco mesh. "Add-on unit" is what written on the box. This is exactly my scenario: I have found that I need one more Deco unit for my home, so bought one M5 in addition to two M9 I have.

In my case I always had my own router (mesh or non-mesh) plugged in by network cable to the ISP router in bridge mode, so that my router acts as router and not AP (my router provides the IPs) and with the ISP router wifi disabled. This gives much more control of my network and I have a wifi network that is "fixed", I can switch ISP easily because my wifi will never change and I do not need to update any home device, unless I really want to.