@Scuba_Diver I have just discovered a downside - the IOT network is not on an isolated VLAN, unlike the guest network. 

I was about to play a YouTube video on my phone (private main network) when it asked if I wanted to stream it to a TV on the guest (IOT) network. That would have been embarrassing as my guests would wonder what was happening. 

This would not be possible if was the actual guest network, which is isolated from the main network. 

  @eppy I have just tested the workaround suggested in the cross referenced thread: Reported Issue and Workaround: Guest Network works only on Main Deco - Home Network Community

What I did was:

1. reboot the ISP modem

2. rename the IoT network

3. disable the IoT network

4. enable the Guest network, give it the same SSID and password as before

The clients seem to be able to connect to the Guest network now.

  @Scuba_Diver Thank you for this, this is what i had to do.

as for TP Link,

The latest firmware update (1.8.5) has broken a fundamental feature: the guest Wi-Fi network.

After nearly a year without a firmware update, a release that breaks a core feature is simply unacceptable and points to a major failure in your quality control process.
 

It's clear that this build was not properly tested before being pushed to the public. As a result, many users are now left with a broken network function that is essential for maintaining network security and for guest access.
 

We need an official response and an immediate fix. Please address this significant quality control oversight.
 

  @Scuba_Diver 

Scuba_Diver wrote

I have the same situation, with two M5 connected via wired backbone. 

The workaround suggested by @eppy worked for me as well:

1. rename the Guest network

2. disable the Guest network

3. enable an IoT network, give it the same SSID and password as the initial Guest network

Unfortunately I have now discovered that the IOT network is *not* on an isolated VLAN, unlike the guest network which is. When I am on my private (main) network and start the YouTube app on my phone, it immediately offers to cast to the TVs on the guest (IOT) network. This is unacceptable. 

I have now turned on device isolation for all the Smart TVs as a workaround, but this means that my guests can't stream to them either. 

I am surprised and disappointed that the TP-Link engineers following this thread and my ticket didn't point out up front that the IOT network is not isolated from the main network. 

This firmware is buggy and we urgently need either a rollback capability or a fix for this serious problem. 

  @eppy I am now running the initial setup, with clients on the Guest network. I had to restart the ISP modem to make that work.

However, the issue is still randomly reoccurring from time to time.

Therefore, I have to reboot the ISP modem every time so that the clients can use the Deco Guest network.

I also agree that using this workaround is not sustainable and a fix needs to be provided for Deco M5 running on 1.8.5 firmware and using Ethernet backhaul.

  @David-TP 

Thanks! I have upgraded to the beta version. The clients connected seamlessly to the Guest network after the upgrade.

I'm currently monitoring and will post here if I notice anything out of place.

Hi David, Thanks for your help with this issue. I tried to test the beta firmware you sent over, but my Deco system has already updated to version 1.9.0. Could you please confirm if the fix for the guest Wi-Fi issue in the 1.8.5 beta version has also been included in the official 1.9.0 release? I haven't had a chance to test it myself yet, as the previous problem caused significant disruption, and I need to schedule a time to re-test it. As a result of this experience, I have now disabled automatic updates and will not be turning them on again. I'm also waiting to see what others on the forum report about the new 1.9.0 version before moving forward. I appreciate your assistance.

My system is Deco's M5 hardware version v3, i have 6 running.

@Registered55 Curious to know which region are you in?

The firmware versions available might also be different based on the region you are in.

I'm in EU and I have the same hardware version 3.0. However, firmware 1.9.0 is not available for download.