I've also noticed that devices that are close to router (~3m) maintain their sessions for hours and hours, while devices in the other room reconnect once in 20-40 min.

@randomactions 

Under 'normal' circumstances, the 2.4Ghz SSID will reach farther than the 5Ghz SSID. What you are describing is the phone losing the 2.4Ghz SSID (could be many reasons), and going back to Cell, and then aquiring the 5Ghz SSID.

While 2.4Ghz will go a longer distance, it also decays faster. There could be a situation where the 5Ghz SSID is stronger (5Ghz power does not drop as fast as the 2.4Ghz signal does over distance from the router). Then it is possible the phone will switch by itself to the stronger signal it detects.

Also, there could be 2 other mitigating factors involved here.

1. Interference, plain and simple, some happens on the 2.4Ghz band due to home cordless phones (some models) or even microwaves. Outside sources too, like motors, all can cause this. Also 'walls' between router and device can degrade the signal.
2. Smart Connect itself. It might be doing 'load leveling', that is keep the number of devices on each SSID equal, but that generally is done when connecting, not once connected, but I am not sure if this feature is enabled by TP-Link either?

The 'best' fix for this, on the device FORGET the 5Ghz SSID. That way it will only connect to the 2.4Ghz SSID, even if a 5Ghz SSID is available.

@IrvSp SmartConnect is enabled. And when it is enabled there are a single SSID for both 2.5 and 5 GHz

@randomactions 

I know that. What you have to do is DISABLE Smart Connect. Then connect to the specific SSID you want. Should stay on that SSID unless the signal becomes too weak and it switches to Cell service.

It still sounds that possibly the device is dropping it though.

Have you tried without Smart Connect being used?

@IrvSp 

thanks for your suggestion! I've tried turning SmartConnect off (now I have two different SSID - for 2.4 GHz and 5GHz). Phones still seem to reconnect from 2.4 to 5 and back once in a while. Not sure if this is better compared to when SmartCOnnect is on. Will monitor more.

Thank you!

But I am still wondering if SmartConnect is designed that way when a phone one-two rooms away from the router would loose wifi signal for a while and then reconnect. Because I though that switching between 2.4 and 5 GHz will happen seamlesly when SmartConnect is on, Isn't it the case?

@randomactions 

Answering both replies above.

First, if it IS switching, even with Smart Connect OFF, it means the phone, for whatever reason, has lost the signal for the SSID it was on.

Smart Connect should not really switch, but there are differing implementations of Smart Connect. Depends on the router vendor too. Some can be 'dynamic', that is they will move devices after they've been connected to another band to keep the band usage even. I don't know if TP-Link uses this option, but I suspect not.

One problem you might actually have is the phone itself. Some can switch to the highest powered SSID it sees. Rare though I think? One thing that appears to be happening is your phone is losing a connection. Both with and without Smart Connect enabled it appears.

Under Smart Connect, yes a single SSID is used. The Smart Connect internals on the router handle which band (2.4Ghz or 5Ghz) the device will connect too. It can tell what band it can handle via some internal manipulations of handshakes with the device. Then it assigns the device to a SPECIFIC band. Without Load Leveling enabled Dynamically, the device WILL stay on that band. Slightly different with Smart Connect OFF. There the device looks for SSID's. It had possibly many to look for, depending on ones you had connected too in the past. In the device you usually have ones for outside your router coverage, and if it is a DUAL BAND devices, more than one for different routers/hotspots. Usually for each router/hotspot there is a specific one you want to connect with (on Windows PC's and others, the device has a setting for the desired band it wants to connect to). In most devices it will try to connect to the last one you were on first.

I suspect the phone has 2 connection settings for the bands, one each. So if NOT on Smart Connect, the 2.4Ghz connection would connect automatically. To make the 5Ghz connection the desired one, you'd need to disconnect the phone and then manually connect to the 5Ghz connection. Might also have to DELETE/FORGET the 2.4Ghz connection.

However, what you really need to determine is now what happens (you know this) but WHY it happens. 3 possibilites here, router, signal, or phone, as the cause. Seems you can rule out the router since the problem doesn't happen close to the router. So what is left is the signal or phone. I don't know the make/model, nor if the phone even has a setting to tell it to switch to the strongest signal? I am not sure if any do, but Cell phones work that way switching from tower to tower as you move. Seems something similar happens to you, 2.4Ghz signal disappears, the phone switches to Cell, and then the phone connects to the 5Ghz band via Smart Connect as the router can't get to it via the 2.4 Ghz band. It will stay there probably as well? Does it stay on the 5Ghz band once it switches no matter where you are? If it switches back to the 2.4Ghz band, well, I wouldn't know why? It would be the phone, unless it seeks the strongest signal, but since Smart Connect is on, it is using the 2.4Ghz credentials? That would imply the router is doing this?

One thing you can do, is get an app on your phone that shows the power of all the SSID's and bands it sees. These are generally call NETWORK ANALYSER'S. Walk around with it on. See if the 2.4Ghz band drops or other SSID's are on the same band and stronger. It will help determining if the problem is a channel you are on is congested causing problems, of if your band DISAPPEARS completely that could indicate ROUTER problem, but only if the signal power wasn't going down the further away from the router you were.

For instance, I use Acrylic on my PC to see the signals:

I have a TRI-BAND router, The RED box on top are my 3 SSID's. Note the POWER (Strength, RSSI) of the signals. You'll see others around me that are weaker.

You'll also see on the right side under the RED box 4 other lines (there are 5 other SSID's list, but when I restarted the app, it did not see the last one list, it is really weak @ 90 so it probablty disappeared before the plotting started?). The line UNDER the RED box you'll notice has a 'break' in it. Don't know the reason, but if that is 'real' (could be an app having a problem reading it) it could be what you'd see for the 2.4Ghz band.

I let the 2nd instance run a little longer and it was interesting:

Look at the ARROWS. The 'PINK' is my 2.4Ghz SSID and the dark BLUE is another 2.4Ghz SSID close by me (actually across the street. I am using the 2nd 5Ghz SSID which is the BROWN line. I suspect that the 2.4Ghz (which my network adapter can work on) lines may not be read correctly? You can see my PINK 2.4Ghz line dips too but never breaks, but always down to the RSSI level of the BLUE line. I sort of discount those breaks.

Anyway, a similar program on your phone might give you better insight as to what is actually happening possibly. In my case, if I were running on the 2.4Ghz band and had it drop on me, using this tool, and seeing it, I'd first change my CHANNEL from 7 to something lower as 8 for the other one overlaps part of channel 7. So far though NO DEVICES connected to my 2.4Ghz (like my wife's phone, streamng TV) has ever dropped the connection that I know of?

@IrvSp that is all nice, but apparently my router model is mentioned in the first message. So the question is still there. Is this how Smart Connect function supposed to work. I would prefer a Yes or No answer at this point.

I would also really appreciate a reply from TP-Link