@Takuiq 

Answers to your questions:

1) Networking switch should be OK if it supports Deco Ethernet backhaul. Not all switches do, and for managed switches you might need to make additional configuration settings changes in them to enable support for Deco Guest Network.

To avoid all that hassle, if you have an option to connect Satellite Deco with Ethernet cables directly to Main Deco - use that option.

2) IoT network is useful for older devices that only support WiFi 2.4 GHz and do not understand dual band (one SSID, multiple WiFi bands). Devices such as water pump, some older security cameras, smart thermostat that is few years old, etc.

If you find that you have one of those devices and they can't connect to Main Network - try connecting it IoT Network in its default WiFi 2.4 GHz only configuration. 

Other than that, if everything connects to Main Network just fine, you don't need to enable and use IoT Network.

3) I manage my smart devices through their apps: Tapo devices through Tapo app, Kasa devices through Kasa app.

4) I don't understand this question. Main Deco is a router on its own, it does not need another router. If you can share those "schematics" you are talking about or give more details, and also explain to where exactly Main Deco is connected with Ethernet cable, I may be able to understand your question.

Generally speaking, if it works, just keep it as is. If it works, it means everything is fine with network setup and connectivity.

  @Alexandre. 

Hi Alexandre, thank you for your answers!

Briefly:

1) hmm how could I understand if it support it or not? I mean, I don't have any kind of interface where I could login, I've just bought this: TP-Link Ls105G Ethernet switch to where I connect the main unit and the others 2 Ethernet cable... Internet works, but if it support backhaul... I'm not sure? 😅

2) i see.. well I've connected them to it now... Switch between nets it's a pain (I've some mini4 sonoff and some meross thermostats), even if I go inside them and change the network, they doesn't work... Everytime I should open the wall and pair them manually... I'll try leaving them in the iot net for now... If I've understood well it's just another 2.4ghz net, leaving the main network in its 2.4/5ghz without splitting it, right? It could compromise the general performance?

3) generally I add them also on their main app because they have many options but to use them with alexa, the tapo app doesn't have a working connection/api/ whatever...  While sonoff and meross yes, so... But what's weird that I've discovered is: if I add my sonoff first to tapo app, then pair them again through sonoff app and then alexa app, they still also works with the tapo app...

4) hmm my question came from this backhaul wiki from tplink: https://www.tp-link.com/us/support/faq/1794/

Basically, I came from a tenda mesh system, cheap, working but often my smart devices got disconnected, so I decided to try this mesh from tplink. While talking with tenda's assistance, they told me it had to connect through a router even if it was working without problems without it. So it let me thinking was a condition necessary to make it works better? Also in this tplink page any schema shows a router in between, so I thought they were right....even if I've tested both system myself and another router was useless as you stated, so i was pretty confused...

Also, they told me to use them in AP mode for a better stable management of iot devices... it's also true or just a legend? Because in ap mode many features are lost so I would not use it if not really necessary....

Thank you for your time!

  @Takuiq 

1) You only need to check in Deco app, under "Show List," to see if switch is supported. If switch supports Deco backhaul, all Satellite Deco connected to it will report Ethernet backhaul, if switch does not - some or all Satellite Deco will be on WiFi backhaul even if they are connected to the switch with Ethernet cable.

Example from my home Deco mesh:

Bedroom Deco is on WiFi backhaul, Office and Study Deco are on Ethernet backhaul.

2) IoT Network will not compromise the general performance. If you have time, move all devices that need WiFi 2.4GHz to IoT Network and turn off 2.4GHz on Main Network:

This could improve performance on Main Network, because that will not allow devices connected to Main Deco switch to low speed 2.4GHz.

3) Whatever way it works for you, I don't have recommendations on that topic. Just my preferences.

4) I still need a bit more info from you. You said "I've connected the internet cable directly to the main unit" - so one end of Ethernet cable is connected to Main Deco. Where other end of that Ethernet cable is connected? Is it connected to some sort of a box? 

It is possible that you have modem provided to you by your ISP and in that case it fits the schema, where it says modem/router. Meaning "modem or router."

  @Alexandre. 

Hello again.

I've noticed a weird behavior: basically I've added different iot devices as iot devices inside the deco system. In their preferences, they are set as auto for which unit they should connect and also for frequency... Even if I choose a specific node and bandwidth, after saving and going back inside its preference, it's again set on auto... there's a reason? It's a bug? I've attached a pic showing all devices still connected to main unit... weird

For the backhaul topic, then it's working. What's weird is that one unit tells me I've a slow speed backhaul, not the other.... Same cable cat.6...

As for point 4: I've a cable coming from the the outside, in my case I've a dedicated antenna which provides a cable that I could connect inside a router or whatever it connect to it...but if it was fiber, the concept here  is the same: they bring inside your home just a cable from the wall that could be connect to a router, it's different there?

  @Takuiq 

Connection Preference feature is recommendation for device. Device does not have to follow it, and older devices often don't. Still, there might be something else with what you see and it might be related to the switch.

Do this test: connect Satellite Deco directly to Main Deco, not through the switch, bounce IoT devices connected to IoT Network and see how many connected to Satellite Deco. 

>What's weird is that one unit tells me I've a slow speed backhaul, not the other.... Same cable cat.6...

You may need the replace that cable. Slow speed backhaul message means cable issues. Before replacing cable, unplug it from both ends, wait till Deco app shows that Satellite Deco on WiFi backhaul, and then plug back Ethernet cable and make sure ends do not feel loose. 

>As for point 4: I've a cable coming from the the outside, in my case I've a dedicated antenna which provides a cable

Your antenna plays role of the modem, so when looking at networking diagrams from TP-Link where it says "ISP modem/router" that means "ISP antenna modem" for your home network. You Mian Deco is router connected to modem (antenna). Keep that configuration, do not change it to Access Point mode or Deco mesh might not work.

  @Alexandre. 

Thank you, I'll try your recommendations. Fkr the cable well it's brand new so it's weird...the technician came with a whole 50m cable and just cut it on the spot.... 

As for the iot devices, in the faq provided inside tplink app, it says that devices with hub sometimes are unable to determine the deco unit and therefore are showed under the main one....which is weird anyway in my opinion but well... 

Also I've noticed that in iot mode, the device get a ipv6 and no wifi signal is showed near it, while if I add them as normal matter device, it has wifi signal, an ipv4 but I can also choose which band and unit they should connect... wondering which setting is the best frankly 🤷