@Dan_C. My router is a Fritzbox FttH router. To test your theory, I've installed DHCP on my home server and disabled it on the router. This gave the same effect (few hours good) On the DHCP server, I...

Just got home and unplugged the APs on the 1st and 2nd floor, now the AP on the ground floor has no issues, speedtest >450Mbps. Some communication issue between the APs?

I've just got the Deco X20, for my 500Mb fiber connection. My network looks as follows: Router - TP-Link 16p Gigabit unmanaged switch - 3x TP-Link Deco X20 (one on each floor, wired in AP mode). All...