The same issue with my Deco. Configured M5 device as main and additional 3x P9 ones.

List of connected devices dissappeared and not showing at all. 
 

Honestly, I expected less issues with configuration of Deco devices for that money. I have spent hours already to make network working.

TPLINK guys, please, figure this issue out for me.

Same issue with my iPhone and whole house mesh system but switching to carrier data from Wi-Fi fixed mine as well. Has to be app related one would think.

@ElsonThomas This kept happening to me.  I rebooted. I reinstalled.  Almost ready to box it up and return.  Then after another successful install with no devices showing, from the app, I rebooted just the main Deco, not the others.  It took forever.  So much so that I went outside to play basketball with my daughter.  It bothered me so much, I kept wanting to go inside to check it!  UGH.  We played for about 45 minutes.  When I came inside, all the devices were showing.  It also said all my other Deco's were offline but when I clicked on them, they came back online.  So I'm good now.... for now... 

@ElsonThomas and all...I had the exact same issue. Everything worked fine, but I could not see any active clients. My status was blank, when it normally would say "online." My wifi info was blank. None of my Decos showed up in the app. 

Coincidentally, I only noticed because I was planning to add a 3rd Deco to my network. I had just finished building a pole barn and a year or so after I bought the 3 pack, I was finally ready to install. Anyway, it allowed me to install the 3rd Deco. After the setup of the 3rd Deco was done and good to go, I returned to the app and wala, everything was back to normal. I could see all my connected devices and everything. 

Prior to adding the 3rd unit, I attempted to do a reboot, unplug the units and upgrade firmware...all to no avail. It wasn't until I added the 3rd unit that everything returned to normal.

I wonder if factory resetting only one of your Decos and reinstalling it might correct your issue.

Antonio

@Tony I am going to return my Deco M9! It's frustrating not showing my devices. No way to port forward those devices. Try all different ways as suggested, still not working! Your Deco App is a loser, I am sorry to say that!

@JuraUrban 

@TP-Link 

Could you please figure it out?

Are you looking professional or not?

You did such a good work, but now everything is broken. I;m not able to use the system anymore. I want to create a fwd rule but if the device is not showing then is nothing to be done.

Thanks and BR

Madalin

@MadalinRSK 

i am experiencing the same issue, I have my Decos in AP mode and some of the clients have disappeared, I restarted and everything but that doesn't fixed it. 
 

I hope to see a solution soon, regards! 

My DECO's are back 2normal now after TP-Link new firmware and enabling the ICMP Feature.