We have had a p9/S4 since 2019 running in access point mode hooked up to Verizon Fios Gb service (recent add-on). Please note I have a fully updated system. Here are my observations.

The Android phone app has never worked as expected.  No version has ever been able to reboot the system. or obtain speeds.  It often shows everything as offline even when it is online.  The application is at best pitiful.

The browser access is even worse than the app.  It cannot show the topography ever and the number of errors that the units produce is phenomenally high.  The CPU is always at 100%.  The issue is that the browser does not say which unit is creating the errors or which CPU is pegged at 100%.  So, I cannot fix it.

Numerous speed tests over the years show that WiFi and wired connections never reach full capacity.  I have checked the cables over the years and they are all CAT6 or better.  Even the master P9 plugged into the Verizon router cuts the speeds to half.  

The upside is that while they seem wonky, the P9/S4 mesh works.  The devices at the far ends of the mesh are slowed but they all work.  While it is annoying that TP-Link has never fixed things since 2019, we can get by.  The newest firmware updates reduced the stability of the Android App and did not fix the errors.  I know these are not big sellers and are old, one would have thought that in 6 years the issues could be fixed.

Keep in mind, that this is not just from one day of use but hundreds of hours of use.   At least, fix the app.  It remains an unmitigated disaster.