@TeddySwede after spending a month in "dialogue" with the support team I can draw some conclusions. 
 

The P9 does indeed make a choice of connecting point, which doesn't give powerline any priority at all. Consequently it happily codes a flaky WiFi signal over stable powerline supported connection. And it is not possible to override the poor automatic choice manually. Ergo: my networks become flaky, due to poor software design from TP, and there are no fit to understand, or brave to admit and solve the issue

-> the P9 may very well be a total waste unless the network consists ONLY of P9s. In my case this was a waste of money, and worse over a waste of considerable amount of time spent trying to provide them with constructive issue reporting to help them meet reasonable expectations and improve their product. 
 

I give up.

  @hiuuq 

I have note heard anything despite being very clear and providing all thinkable documentation why there appears to be a solution design issue which should be possible to fix by a software update. Quite disappointing.

thank you for your answer. I don't think there's another workaround till the feature is available.

I really need this to be a feature. Why this has not been done yet I can't undestand. It's actully shocking that something that should be simple is still not preset years later when it is a real world problem for people. I'm having the it with wifi backhaul between deco's and wifi cameras. I have a S4 in a shed and 3 M4's, despite the fact there is a M4 in the garage near the shed the S4 connects to the main deco in the top of the house will barly any signal and the camera in the shed connect to the very furthest point to the M4 in the front room. Madness. Also the fact the deco in the front room connected via the lan still keeps switch to wifi backhaul is really hacking me off with no way to force what backhaul to use.

TPL needs to get its act togather but they won't. Time we all look at other brands I guess which is a shame.

  @Driftingforlife 

any update or more recent thread to this?

my study deco connects to my main one which is miles away with the absolute worst signal, completely bypassing one that's much closer. Very frustrating. 
 

  @Savv 

I'm having this functionality in my Deco app. I have three Deco M5.

For the two Decos that are not main I can specify which other Deco it should connect to.

For all conected devices I can specify which Deco it should connect to.