@Shev You'll see from this forum that a lot of us have this problem.  I'v done Factory Resets on units, and still the problem returns, even if the unit is standing 2 metres away from the main P9 in router mode in line of site.  It's infuriating, as there is no other WiFi solution other than mesh I can use in my house. 

@tsm2020 I didn't retun mine but have stuck with them because when they work the mesh network is brilliant. I have put them in access point mode rather than router mode now and that appears to have made things more stable. I haven't noticed a disconnect since I did that so that could be an option for you.