I am having the same issues. Having followed all the above steps. I still have not resolved the issue. The only way I can get devices online is to either assign them a static IP and manually setup the DNS and gateway or turn the device on, reset the DECO main unit then go into the add and wait for it to turn up. (The broadcast name of the device is always shown incorrected and has some random name instead, so I have to check MAC addresses) and then add it to the reserved address list. 

Would love to know what is going wrong as it has taken over 3 days of troubleshooting to get my devices back online and whilst I havent solved the problem I have a patched work around.

Anyone had any experience and how to solve this at all?

@Kevin_Z

I have the same problem.

I recently bought a 3 pack tp-link deco p9. Everything works fine except for my living room lights. The Magic Home Pro app, as well as Alexa api can not reach the device.

Not being able to switch on/off the lights is really annoying.

Please note the system works well and stable with a tp-link C5 router as well as with a Huawei q2 pro mesh system. 

What I tried so far: IPv6 on/off, fast roaming on/off. Each time I reseted the device. It works a few times, and then even if I see the device in the deco app, it is unreachable by the app and Alexa.

I also enabled port forwarding. Same thing.

The only way to reach it is to add to the black list, thern remove it. I can turn on/off the light a few minutes, then it becomes unreachable.

What shall I do?

@Kevin_Z  Hi Kevin.... i have installed a new wifi mesh network with Deco P9 speed is good all is perfect but i cannot pair my sonoff wifi switch to the new mesh network ....i suppose that the problem is 5GHz frequency ....is it possible to switch off 5GHz signal in Deco P9 ? Thks 

@Dado68 

Hi, no, on P9 you can't turn on/off different speeds. I really hope is possible do this after new update/new firmware.

To solve your problem try on this way. At first, check if you see if the sonoff device is on the Deco's network. If it is on your deco's network, add it on blacklist (yes, on blacklist) and wait about 1 minute. After, remove it from blacklist: the sonoff device would work fine. I try on this way and all works fine: i hope it is fine for you too. Let me know if it's work fine.

@Kook

Try on this way: At the time the devices are connect (add to blacklist, remove to blacklist), give them Address Reservation.

Happen de same to me, eruthing worked fine during the last year, and some devices started to have prblems with the internet. by the way this dvices work perfectly whhen I use my Hot Spot Wifi form my 4G celular.

@Kevin_Z Having a similar issue...

In my case, i have a tp-link deco m4 system with 2 Ring Stickup Cameras, and they are intermittently online and mostly not sending alerts and not available for live view.  I would estimate they are online/available randomly for 2-3hrs every day.  I've run through all of the "Ring - Fixing Your Devices Falling Offline Repeatedly by Adjusting Your Router" steps:
1.  internet speed is fine (15/10)

2.  signal strength is fine (-60,-40)

3.  limited network to 2.4ghz, and can see it operating on channel 8

4.  setup dhcp reservations for the stickup cams and base (fixed IP)

5.  all firmware up to date on the cameras and tp-link router

6.  no special characters in my ssid.

7.  disabled mesh for the stickup cams.

8.  fast roaming disabled.

9.  prioritized ring devices on the network.

Tried addressing issues in the "Ring - Fixing Your Devices Falling Offline Repeatedly by Adjusting Your Router"  post, with limited success because tp-link doesn't make these settings visible or configurable.  Are any of these protocols/ports incorrect for the M4?

The same setup ran fine on my older tp-link archer router (non-mesh), but the cameras were so far away, the signal strength wasn't great so i upgraded to a cheap mesh wifi system (TP-link deco m4, set of 3).  All of my other devices on this tp-link wifi network work great (better than before), including an ecobee, 5 year old ipad and 10 year old wifi e-readers.  Everything but my stickup cams.  If they don't sent me alerts, there's no point in having security cameras.

I can see many others having the same problem.

What am i missing?

@Kevin_Z Hi ....thks for reply ...solution not so easy ...in Deco P9 you cannot manually switch off the 5GHz frequency ...it seem that Deco network itself decide when to use 2,4 or 5 GHz ....only this model .....probably need to wait a firmware .

It is possible to decrease this problem adding and removing devices from black list lik other customers suggested .

Rgds

Davide

@Dado68 

Hi, does your sonoff swich have a ESP wifi chip?

Hi to everybody,

I've just installed my new Deco M4; the setup was a piece of cake, and the WiFi range covers my house without any problem.

But I have unsolved difficulties with 2 of my webradios, the 2 others working flawlessly

One webradio detects not one, but 5 Deco networks. I am able to select one, enter the password code, and it's OK. But, after powering down and up this unit, it is unable to connect to the previously selected network. 
It conducts me to the screen choice, I select again the "good" (previous) one, and after a while it works.

But after powering down again, the problem comes back

It's not very convenient fot a system supposed to create one network !

Please help

Thanks in advance