@DGMike same here. 

I installed my old router back and put the 3 deco's in AP-mode. 

They work great now. 

Tip: do NOT put them in a power brick. They MUST BE directly in the wall outlet.

After i did that, I haven't had any problems anymore.

System is not useable with router function and 2 AP.

TP-Link spits in the faces of their customers. Never again for me.

@ReneP 

I'm not an expert on this--but there are a couple of things about how you power the Deco that could be at play when trying to use powerline as backhaul carrier.

First, in the US, you have a 3 wire service, two hot wires and a neutral.  Hot to neutral = 120 V which is what you get at the outlet and hot to hot = 240 V which is used for larger equipment such as dryers and HVAC.  

If the outlets for each deco are powered from the same Hot to neutral, then powerline should work okay.  If they are not, then you don't have a powerline connection. 

Power strips are typically provided with surge protection which is a good thing, but I suspect that the surge protection might also act as a wave trap and block communications between Deco's.  

As of right now, I'm still using my Deco's as 1 router and 2 AP's and I think they are working okay.  I know I'm having an issue with my internet service which is due for repair tomorrow.  Hopefully, that will be fixed and I can evaluate my P9's fully.  

@DGMike Thank you for your reply. I live in the EU. My house has no grounding in the outlets, except for utilities. I have only one phase of power.  Newer houses have three phases. Bridging can not be done. APs on  different phases mostly can not communicate with each other by power line. 

Some systems use the earthing wires because this has less interference from utilities like micro wave. Not sure if P9 uses this system. APs are stable for me. Range is good. 

Functionality of the router functional is minimal. Very happy with my Fritzbox.