@Directy 

People who develop software for smartphones are usually very good in making wireless connectivity work right. So, it would be easier to focus on smartphone first, to figure what is going wrong with your Deco mesh.

In general, with proper mesh configuration and correct smartphone behavior, it should work this way:

1. Turn off/on WiFi on smartphone - it should connect to the mesh node with strongest signal;

2. Walk around the house - as long as WiFi signal is good, smartphone will stay connected to the same node. When signal drops below threshold, smartphone will rescan WiFi networks and connect to the mesh node with the strongest signal.

I am going to speculate that you have strong overlap between WiFi signals from different Deco E4, so that smartphone has harder time to choose the right node, because more than one gives good WiFi signal. Ideally, in locations that are relevant for you, i.e., where you spend most of the time, you should have just one E4 node with strong WiFi signal and rest with substantially weaker WiFi signal.

Recommendations on how to improve your Deco mesh.

1. Turn off 2.4GHz WiFi in Deco mesh.

That will leave just 5GHz band for your smartphone to use at your home network. WiFi 5GHz signal goes shorter distances, by design, which will improve ability of your smartphone to pick the closest node, and to switch to the closest node faster, when you walk through your house.

2. Install WiFi analyzer on your smartphone.

That will give you clear picture of WiFi coverage in your house and what Deco node your smartphone is connected to, in real time. Deco app gives that info with delay.

I use WiFiman from Ubiquiti. This is what it reports to me about three node Deco mesh I have:

As you can see, in that place of my house where I ran this test just one node has strong signal, and that is the node smartphone is connected to (Linked).

With the assistance of WiFi analyzer you can review true WiFi coverage in your house, and perhaps find the way to place Deco nodes differently to achieve more optimal coverage.

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Even if you decide to replace Deco with different brand, these recommendations will still be valid.

Directy wrote

ok but ip cam does not move around my home... and they have the same problem...

@Directy 

They have appearance of the same problem, but root cause is most likely different. 

Does your ip cam support 5GHz? Because, if it does not, it could mean its WiFi connectivity code has not been revised since 2009 at least. Not very many household mesh systems were running in 2009.

IP cam might not know how to efficiently deal with WiFi mesh, meaning multiple nodes behind same SSID.

my ip cam are quite new d-link model and yes...  does not support 5g but my iphone 12 and my new ipad support 5g but i get the same problem

TADO iot, philips hue, shelly support or not 5g (i dont know) but the problem is that they connect to a random DECO and i have a lot of disconnection problem.

@Directy 

For devices that support WiFi 5GHz consider recommendations I've provided, at least to see if they can help. 

Also make sure you haven't disabled Mesh and Fast Roaming settings in Deco app, devices such as iphone and ipad can use these to decide which mesh node to connect to. 

@Directy for wifi devices that don't move I disable mesh technology for those devices only. This solved a few issues I was having with static IP lights