I've confirmed that the 3rd floor P9 is updated to the latest firmware by removing it and temporarily establishing a new network with it. I've now reset it and added it back and it seems to be working for the time being, I was able to get a speed of around 50 mbps in that room, which is acceptable.

Are there any known issues with using P7's and P9's together? It's my understanding that the P9 creates a better powerline connection, which is why I'm using those for the main Deco and the one furthest from it. Is this correct?

After a few days of stable operation the 3rd floor P9 lost connection to the mesh network and would not reconnect even after repeated restarts. After fussing with it for about an hour trying to get it to reset in order to add it to the network it's finally back online. I'm fed up. Can anyone recommend a better system for my application? Thanks.

@TP-Link 

-They are set up in router mode. I'm using the parental control feature and would like to keep this function. Would access point mode result in better performance?

-Yes, when the system is working the PLC indicator in the app is on for each Deco. The wireless connection is weak, usually yellow on the third floor.

-Yes, when the 3rd floor Deco loses connection it flashes red. On of the second floor P7's recently did the same thing, though the P7's seem to be easier to reset.

-Originally I had a P7 on the third floor and it was also inconsistent. I was hoping the P9 would offer a better powerline connection. It's certainly a tricky location.

My main frustration is that I can't seem to get this system to work consistently. It will work well for a few days, then a unit inexplicably malfunctions.

Thanks!

I went ahead and ran an ethernet cable up the exterior wall of the house to connect the 3rd floor Deco directly to the main router. While it's not an elegant solution, I wish I had done this sooner. Instead of adding the wired Deco to the mesh I set it up as a separate network, which the app allowed me to do easily, and now I get full ISP speed up on the third floor. And since our kids are locked out of that new network they won't be able to suck up our bandwidth like before. I think the lesson here is that there are limits to what a powerline mesh system can do, especially in tall concrete structures with lots of users, and once you bump into that limit you just have to find a way to run some wire.