@David-TP Allow me to answer. I just received, configured, and returned a Deco XE75 Pro mainly for not allowing me to disable the DHCP Server. In my network, the Xfinity Modem can be a DHCP Server, but I disabled it. My current mesh has been Gateway Orbi for a number of years, and I also have the DHCP Server disabled, I'm pointing at my own set of primary and secondary DNS Servers so I don't have to go to my ISP for that, and I also use port forwarding.

In any case, my DHCP Server is a primary PiHole server (I have 2), so I can see which queries for which devices are being allowed or blocked. If any other server was providing DHCP, all I would see was my router's IP Address. I also have IP Reservations for almost every device in my network. 

TP-Link should offer a real Web Management app, as a phone app is far from ideal for that. I tried yours, not good.

I hope that answers your question. 

I agree, there is something is wrong with Smart DHCP - with it disabled, some devices have trouble obtaining DHCP addresses from my pfSense router. With it enabled, it works - but that seems to indicate to me that Smart DHCP is manipulating the network somehow.