Look for other threads, there are multiple devices with same symptons, mostly tablets and mobile phones: they show the wifi as connected without internet, the device connects but after some time (minutes) looses internet connectivity - different hardware models, android versions, manufacturers. Tp-link just mentions this is probably a compatibility issue between the deco's and the mobile device but, there are so many different devices experiencing this issue with different deco models (M4 for me but you will find issues with M5, P9, etc.), that looks like something common to all the decos firmwares. On all threads, when turning on the former ISP/router wifi and connecting to this one instead, the device with the wifi issues connects and stays connected fine. This cannot be an isolated incident anymore, it has to be a particularity of the tp-link mesh implementation.

A few days ago my dad was "experimenting" with it and unplugged the 2nd Deco (so only the main Deco was powered) and after that, the issue was completely fixed. So it's definitely a problem with TP-Link's mesh implementation. The problem is that I need to have the 2nd Deco powered on because our outdoor security cameras cannot connect to our WiFi.  So I plugged in the 2nd Deco again and a day later the issue came back. Our ISP didn't give us any router, so I can't plug it into an ISP-provided router. TP-Link should definitely fix this (if it's a firmware issue).

@asciiid  Just a quick Question.   Is the Deco system in Router Mode or in Access Point Mode?

@Stefan_A The Deco is in router mode. But I tried to use my old Asus router and connect the Deco to it and change it to AP mode, but that didn't solve the issue either. It definitely seems like the Deco is the problem.

@asciiid I'm having the same connectivity issues. Apps had a difficult time connecting to the internet with the TP Link M4R. I have to disconnect and connect again after a few minutes in order to get back online even though I am connected to the internet, and the internet is working just fine. This is getting really frustrating. I have tried all the possible solutions and configurations but to no avail. 😔😔😔

I have no idea what is going on, the only reason why I'm not going back to my old router is the Deco's awesome range, it's really good (at least in my case). But since most of our devices are experiencing this issue, I'm not really sure what I'm going to do/what should I do, this is so annoying. I even tried to disable 5GHz and just keep 2.4Ghz, but that didn't fix anything either. I'll probably contact the seller, I may be able to get a refund, although it's probably too late to file a refund request. 

@asciiid I'm having similar issue with my Logitech circle camera. It seems good for first couple of days but now for last 4 days it keeps disconnecting. I've turned off "Fast Roaming" and still no luck. I even reset my Camera and I'm still facing issue. 

@Stefan_A  I've this same issue and my Deco5 3 units system is on router mode, conected to the modem by LAN Cable (main Deco only)... is that something to change?

@asciiid Add 1 more M3 system to this issue.

I bought the system for my pastors, and it worked fine the first couple of weeks, now they are regularly complaining about their devices being connected but being unable to browse, use apps or even chat over Whatsapp and Telegram.