Incase anyone else has been sent a new firmware to test on this matter, I thought I'd give my feedback/update.

After applying the firmware update supplied by TP-Link to my M5 Decos, the problem has seemingly got worse :(

Where I previously had one wired in device incorrectly listed as being on the GUEST network, I now have two. In addition to the HP Printer, one of my Windows 10 PC's is now also shown as being on the GUEST network.

Both are are shown as "Wired" and have an IP Address of "UNKNOWN". Both are functioning correctly on the main network so clearly are not on the guest network.

Anyone else been given a new firmware to test?

@NeilF 

On that Windows 10 computer which is on GUEST network, please run ipconfig /all and share its screenshot. May help with understanding what's up. Should show device IP address.

@Alexandre. 

At the moment NO devices are defined to be on the GUEST network. ie: All wireless devices are on the main (WIRELESS_MESH) network.

For some reason, initially one, now two, hard wired devices have started appearing on the GUEST network incorrectly according to the Deco app. They are NOT on the GUEST network as they are connecting to devices on the main network correctly.

The HP Printer automatically gets its IP address set.

The Windows 10 PC is defined as having a static IP address if 192.168.0.100.

Another similar PC is defined as having a static IP address of 192.168.0.99, and that does correctly appear on the main network.

Clearly, it's impossible for a wired device to be on the GUEST network... And these although shown to be on the GUEST network are not. It's got to be a software reporting error?

@NeilF 

You have multiple devices on your network with static IP address out of DHCP range. Did you configure static IP addresses on devices, or through IP address reservation on ISP router?

I think my router doesn't allow to pick IP address for reservation out of DHCP range, but yours might be more flexible. That's why I am asking this question.

@Alexandre. 

The devices in question have been defined as static IP address on my ISP's router for years, without problem...

Router's IP is 192.168.0.1. The DHCP range is absolutely fine. Fixed IPs for devices are defined as:-

PC X: 192.168.0.99 (wired)

PC Y: 192.168.0.100 (wired)

Deco 1: 192.168.0.250 (wired)

Deco 2: 192.168.0.251 (wireless)

Deco 3: 192.168.0.252 (wireless)

All other device (wired or wireless) are as you'd expect when dynamically allocated. ie: 192.168.0.x. eg: The HP Printer which is appearing on the GUEST wifi (even though wired) is 192.168.0.201 at the moment...

Again, we're talking about wired device, which are working fine, but which the Deco app is declaring are on a wireless GUEST network...

@NeilF 

I am not saying you do anything wrong, I am trying to figure what is the thing that confuses Deco. 

I have Deco M9/M5 in AP mode, printer wired to M5 with static IP address and TV wired to M9 with static IP, too. I also have few other devices with static IP addresses, such as file server and backup server.
No wired devices reported as connected to GUEST network in my home.

It also appears I have mixed your posts with those from @FlixP. It is @FlixP that shared router DHCP IP range, not you.

For clarity, what is DHCP IP range on your ISP router?

--------------------

Also, if this Deco reporting issue annoys you too much, you could get rid of it relatively easy. In AP mode, you don't need all networking traffic to go through Main Deco. Connect Main Deco to switch this way:                                                                                   

INTERNET --> ROUTER (no wifi) --> Switch --> All my network.

                                                                |

                                                          M5 (main)

and you will never see your wired devices misreported in Deco app again.

@Alexandre. 

I want all traffic to go through the main Deco (AP mode) so I can see the behaviour of all connected devices.

DHCP range of router is 192.168.0.2 to 192.168.0.254.

The fact the new firmware TP-LINK sent to me to solve this problem actually increased the problem, by adding the 192.168.0.100 wired PC into the GUEST network too, suggests to me it's a Deco software issue with reporting not behaviour? That device is otherwise working fine and can communicate with other devices find on the network etc.

@NeilF 

My bad. I thought I spotted the trigger for that issue, and that it is static IP address outside of DHCP IP range. I was wrong, at least in your case it is not so. Apologies for wasting your time.

Yes, it appears to be Deco reporting issue, from what you described. Not sure what triggers it, haven't seen the same on my, similar to yours, network.

@Alexandre. 

It is bizarre, because for month(s) all was fine. And then one day there was my HP Printer on the GUEST list. And it's never left!

The fact a wired device obviously cannot even be on the wireless GUEST network makes it particularly odd such a device is. And even stranger that now after the firmware update (to fix the issue), two are...

Just an update on this.

Seems the problem has been solved. TP-Link (bless them!) logged onto my machine and spent the best part of probably an hour debugging the Deco to diagnose/cure the problem. A test firmware upgrade from them has resolved the issue, so wired devices are no appearing on my GUEST network.

I also had an issue with some devices having an "UNKNOWN" IP address in the Deco app. Seems this is caused when the device is defined (on itself) to have a static IP address. eg: I had a Windows 10 PC and an HP printer both with hardcoded IP addresses defined on the Router AND ON THE DEVICES THEMSELVES. Returning the devices back to automatic IP address allocation (leaving it down to just the Router), now means the IP address is shown in the Deco app.

So... SOLVED!