@cocofyn

Yes, I am using the Android app v3.9.42.

Yes, those are the settings I used (and also various other settings as alternative attempts). I also tried entering the same user creds as used for IPv4. All ended up with the same, or worse, results that I detailed.

I have had to revert to putting the XE75Pros back into Access Point mode, and reintroduce the BT Smart Hub 2. What I didn't mention was that it proved impossible to re-configure the Decos from Access Point Mode to Wi-Fi Router mode in the first place. Changing the mode via the app simply resulted in the "Something went wrong" error that the app is so fond of. The only way I could find to do it was to delete the whole network, then factory reset all 5 nodes, then re-configure via the app and add each node back in.

Going the other way (from Wi-Fi Router mode to Access Point mode) fortunately works fine.

So I'm in no mood - nor do I have the time - to go through the whole process again. It might be different if you can detail a way I can easily restore the configuration(s) I was testing from a cloud backup I made via the app.

  @David-TP 

Right, I've wasted a few more hours reproducing the errors. I can get as far as configuring Ipv6. The app says everything is OK, and displays what look like suitable connection data for both IPv4 and IPv6, but 99% of the time *none* of the devices on the Deco network can get either IPv4 or IPv6 access to *any* external website. Briefly I managed to get my phone connected to IPv6 test sites, and they all reported I had no IPv6 access. Once I disable IPv6, all I Pv4 access is fine. 

I have e-maiiled you a log file. FWIW. 

  @cocofyn 

I've had it working for over a year with the following. But before getting started factory reset all the Deco's, set up only the first connected to the Fibre ONT (in router mode), and then when working set up all the others.

Firstly go through the initial setup with IPv4 only. Use PPPoE, username bthomehub@btbroadband.com , password can be left blank unless it makes you enter something in which case use "bt", and make sure you can connect to the IPv4 internet fine.

This may seem odd but stick with me and restart the router and make sure everything comes back up and connects to the IPv4 internet fine.

Now go and enable IPv6 with the following details:

Internet Connection Type: PPPoE

Sync IPv4 Account: Yes

Get IPv4 Address: Non-Address

Prefix Delegation: Yes

DNS Address: Manual (I use Google's)

2001:4860:4860::8888

2001:4860:4860::8844

Assigned Type: SLAAC+RDNSS

And then save the lot. Hopefully this save will "stick" and pop you back out at the main Internet Connection page. At this point you should see a granted Address Prefix and IP Address. If not then restart your router (really!). If you don't see anything after this then wait a few hours and restart again and you should have one and have full IPv6 connectivity.

Don't ask me why but routers across various brands, not just TP-Link, need a restart to kick the IPv6 connection into gear. Even across different ISPs. And that's just coming from a factory reset on a fully-working setup! In your case though it's a "first time" trying to get this hardware IPv6 enabled directly and ISPs (not just BT) can be a bit slow in assigning your IPv6 prefix to the new hardware.

Once the router has got your Address Prefix and IP Address you should be IPv6 enabled for all devices. And can then add your additional Deco's to the network and they'll pick everything up.

That should hopefully get you all sorted.

  @YorkshireBill 

Many thanks for your kind assistance, but I've already been through your set-up sequence many times (starting with a factory reset, and get it all working wth IPv4 first, etc.). The problems arise as soon as I enable the IPv6 settings and restart the main node. I have tried with various other IPv6 settings too (I use OpenDNS servers rather than Google's). I have got no combination that works - most of them stop IPv4 from working, too - while the Deco app shows apparently sensible status settings for both IPv4 and IPv6, no devices on the network can browse any website, and even my phone says WiFi is connect *without Internet".

  @klipse 

I do appreciate your attempt to be helpful, but configuring a standard router successfully is not my problem.

  @David-TP 

I have to say, hours and hours of my time testing all this could have been saved if TP-Link offered even the most basic of troubleshooting tools to the app, like easy and proper backup and restore of full configurations, Internet connection dropping/re-connection (without needing to restart nodes), and easy collection and reporting of log files, etc. It surely isn't beyond a company of that size, and would even make it cheaper for them to provide support (even for the little support they now offer).