@hamlet423423 

I have problems also with a similar setup. 

My IOT devices - primarily the light switches and smart plugs, also cameras, locks, thermostat - keep losing their connections. This has been going on for almost 8 months and I'm about to give up and replace the whole networking system.

My setup today: Archer BE550 Pro Router setup with mesh devices: 3x RE220 range extenders, 1x RE505X range extender, 1x 500X range extender, 3x cheap no-brand range extenders that do not work with the TP-Link mesh system.

Other items I've used within the setup in the past with issues (described below): Archer AX20 router, 2x additional RE505X range extenders.

I have a larger house and trying to make sure there is a good signal everywhere.

Here is what I've tried so far:
1. Most of the IOT devices that lose their connections are supposed to be used on 2.4G and I figured the main network (which uses both 2.4G and 5.0G) is trying to switch them to 5.0G and that causes them to lose their connections. So I created IOT networks and guest networks that I could connect to with only 2.4G from devices that required 2.4G. The result is that the router does not connect to the internet. The main network connects just fine, but not the IOT or guest networks.

Note: I saw somewhere a few months ago that someone with the same router and different range extenders had the same issue with their IOT network not connecting to the internet. They said they removed their range extenders and the IOT network worked just fine. I haven't tried that, but I do need those range extenders because of the size of my house.

2. I tried using the cheap no-brand range extenders connected to the IOT networks (to limit to 2.4G) and had the same issue as in #1.

3. I tried connecting the AX20 router as an AP wired into one of the RE's and that didn't work at all.

4. I tried connecting the AX20 router as a router (with its own SSID and assigning IP's by itself) with a separate SSID limited to only 2.4G and then the router was wired to one of the RE's. This worked just like the IOT networks in that devices connected to the AX20 just fine, but could not reach the internet.

I'm very frustrated. Does anyone have any other ideas? The only thing I have left to try is to get rid of the mesh RE's and keep the no-brand RE's to extend the network. I want to avoid that as much as possible because the no-brand RE's have no wired connection. On some of the RE's, I have devices plugged into the network ports.

  @hamlet423423 

When it came time to update my network, I looked a mesh and found it was not for me.

I did use a router and extender in extender mode. I used a different ssid than the main router. I used the 5 GHz for backhaul and the 2.4 for talking to the devices. Most of the devices were iot cams of several makes. I had issues with a few dropping from time to time and moving the extender more to the half way point worked best. The extender needs a strong signal to both the router and the devices.  One cam failed the most although it was not the one most distance from the extender. I think a metal cabinet near by has something to do with it. 
 

I did use an AP for a bit but preferred the extender. 
 

my new router works without the extender so I no longer use it plus I have got rid of all but one brand of wifi cams and using mostly wired nvr controlled cameras now. Once in a while I will still lose that one wifi cam. I have had several of that brand cams fail over the years and need replacing. Of the five brands of wifi cams, I have not found any of them that were 100%. 
 

I have my iot on the 2.4 guest network now. Before the iot was on either the extender or guest network. I want to control which band each device uses.