@pcouchman 

You got it a bit backward: in Router mode Deco will restrict bandwidth using QoS parameter. In Access Point mode QoS is disabled, Deco will always provide maximum bandwidth it is capable of. 

Switching Deco to Router mode will not improve speeds you are getting on devices connected to it.

The problem must be elsewhere. For initial troubleshooting, power off all Satellite Deco, connect device you use to test speeds with to Main Deco and run speed tests again. 

  @Alexandre. Thanks for the suggestion.

I powered down the two satellite Decos. The Main Deco gave me a speed test of c350Mbps, which is the same speed as the main EE router. However, when I powered up the two satellite Decos again, the speed dropped back down to c150Mbps. Any suggestions on why this happens?

  @pcouchman 

It is possible that when Satellite Deco are powered on, the device you run speed tests on is connected to Satellite P9. That was the reason I suggested to power off Satellite P9s: to make sure device is connected to Main Deco.

When device is connected to Satellite Deco, it may be hitting speed limits of powerline or WiFi link (powerline/WiFi backhaul) between Satellite and Main Deco. This limit may have been there all time, but you haven't seen it before because your broadband connection was slower than that limit.

Thanks  @Alexandre. 

The satellites are connected via WiFi link (not powerline) so it's likely to be that causing the limit as you say. I saw this post , which seems to explain the issue you are describing in more detail too. Not sure I'll be running LAN cables throughout my house so I'll probably look at connecting devices that need maximum bandwidth direct to the router WiFi and let the mesh cover any remote parts of the house.