I currently have a deco M4 3 pack set up with my Virgin Media Hub 3. The Hub 3 is set to Modem Mode and the Deco system is set to work in Router Mode. I have a 250MBps connection.

I have a 4 port TPLink 1 gigabit Ethernet Switch connected to the 2nd port on the 1st Deco unit and I have used one of the ports on the ethernet switch to set up ethernet backhaul to a Deco unit in a upstairs bedroom via a powerline adapter as the wireless signal there was really poor. The 3rd Deco unit is set up via wireless backhaul in another upstairs bedroom.

At the moment I am finding that the deco unit connected via ethernet backhaul is only providing 100 mbps no matter whether I connect to it via ethernet or WiFi. 

I have read the various posts relating to others having similar issues and tried all the tips mentioned on then to try and get better performance but nothing seems to improve the speed. I know the powerline can handle up to 600mbps and someone mentioned it might be the ethernet switch that's causing the issue as its an unmanaged switch.

I am not getting the full speed from my connection no matter which Deco unit I'm connected to via WiFi, as I have run speed tests that all show a maximum around 170 mbps which is well below the 250 mbps that I should be getting.

Should I try setting the Deco units up in Access Point mode? I only use the 4 port switch as the Virgin Hub 3 only keeps one ethernet port active when set in modem mode. 

If I swap the Deco units to AP mode and then change the Virgin Media Hub 3 back to Router mode, and set the SSID and password to the same custom ones I had set up for the Deco WiFi, will everything just connect again without me having to reset all my smart devices??