@AR-jan 

I'm having a similar issue. I currently have a the Deco M4 system. These recieved a firmware update last night to (Firmware 1.4.0  Build 20200628 Rel. 48846). In the firmware comentary, one of the key benefits of this firmware was clearly listed as Google Assistant integration.

All the nodes are updated to the same firmware.

When trying to link my TP Link Deco account with Google Home, I go to Set up device > Have something already set up > Search for TP-Link, ad select "TP Link-Router"

Here I am greeted with the TP-Link Login page, wher you can enter the your TP-Link ID and password

Then the Authorisation page. Once I hit Authorize, this is where I get the following error message: "Couldn't update the setting. Check your connection"

No TP-Link devices are visible in the Google Home Screen (Alongside other smart devices)

The Deco units continue to provide connectivity, but only operate and regular dumb mesh devices, with no Assistant integration.

I have a ticket logged with TP-Link Support team, who have promised to look into the issue. I have provided screenshots of the error.

It's a fustrating, but hopefully they will resolve this.

If you have any workarounds, please let me know. Meanwhile I note that there is a beta firmware (1.4.2)  being suggested on the forum, to resolve some known bugs with the 1.4.0 firmware, so far I resisted an update to this.

@Myapps any solution? 

@Marcelo77 in the Google Home app get to the page which allows you to select the various brands of smart devices.

From here select "TP-Link Router" -  this will launch the account sign in page.

At this point, go to the top right corner of the page. Here you will find three dots for a new menu. In this menu, select "Open with Chrome"

The sign in page will reopen in Chrome. 

From here sign in using your normal TP-Link credentials. (The same as those you use for the Deco app). 

This will take you to the authorisation page. Hit the authorise button. 

This may take a couple of attempts, or a little while to authorise. 

This finally worked for me. 

@Dreamypuma I tried this. I went through everything like you said and it seemed to work but nothing is showing on Google home. I tried 6 times now. It authorises and then goes to the home app but nothing. No options for tp link or is it showing as a device. I scanned for a new device too. 

I'll keep trying. 

@Walgeon in all honesty it's simply not the most intuitive device to add. I'm sure you have already checked, but are you sure the devices aren't at the bottom of your list (waiting to be added to rooms?)

If the account authorised, when you go to add new devices is the TP-Link account showing at the top of the list along with other authorised accounts?

If so, try asking Google to refresh devices ( I think that's the command)

@Dreamypuma , have just tried your manual. Authentication successfull, connection successful, device not in the list. Any ideas?

@Sunsear glad you've managed to authenticate you account on Google Home. 

In all honesty I've found the Deco Connectivity with GH a bit flakey. Needing lots of work around. Hence why I'm sharing my experience on here. (Hopefully this helps others)

At this stage, have you tried asking Google Home to "Sync Devices"?

Hopefully this will update all your accounts and make the Deco nodes visible on your device list. 

They should appear at the bottom of your list. Thereafter you can add them to rooms.

The nodes don't have individual names, so will appear as three identical unassigned devices at the bottom of your list. 

You'll just need to assign them to rooms arbitrarily. (They all serve the same purpose and don't offer individual control anyway, so this isn't a big deal) 

Good luck. Keep us posted if this works. (It will help others)

Connect successfully my deco m5 to GH no issues there.

What commads do i have available?

Cant find any documentation.