I also have issue with devices dropping off the WiFi, both 2.4ghz and 5ghz devices. It just seems very unreliable in general

Maybe I'm misunderstanding but aren't the deco p7s connected over powerline, isn't that their main feature compared to m5/m9 devices. So when you say 'Deco P7 )))(((Slave Deco P7' that doesn't seem right

Anyway the light on both p7s is always green regardless of their online status in the app. Last night I rebooted the main p7 through the app and this seems to have resolved the issues. All devices can connect fine now. 

When this happens again and I connect a pc directly to the slave p7 with an ethernet cable what should I expect to see for the IP address and default gateway? Should it be different to the current default gateway(192.168.0.x)?

Edit: So the slave deco is showing offline again now. It seems that when I go into the room with the slave p7 this is when it starts showing as offline in the app. I'm guessing it might be something to do with the fact my device is no longer connected to the main p7. Is this normal? 

6 months later and I still have this issue with my slave p7 intermittently showing as offline and wireless devices reverting to the main p7(Updates to the app now indicate which p7 a device is connected to). Does anyone else have this issue? I'm wondering if the overall signal strength coming into the main p7 from the modem is a factor because my wired devices are all fine like my home alarm as mentioned in original post and it seems like this could be only happening during high contention periods like evenings and weekends. I did try running a speed test when this issue occurs and it's always giving me the same results around 240mbps down and 25mbps up. Another possibility is perhaps there is too many devices connected to the slave p7(max 12 during busy periods), it does feel quite warm to the touch especially when streaming from my nas box(wired)