On further investigation it seems to be a weird WiFi issue with my Deco rather than anything to do with the internet connection.

I can maintain I constant WiFi connection from my phone to my Deco when the ethernet cable from the ONT is unplugged (but obviously no internet) and as soon as I plug in the ONT the WiFi connection starts to drop.

I don't get it - but seems like a router problem rather than anything to do with my connection.

It seems that connecting directly to the ONT via PPPoE is causing my WiFi to become unstable - if I go back to connecting to my smart hub or disconnect from the internet then it's fine?

Could it be a device on my network causing a problem?

Thanks for your reply, I'm away from my house at the moment but will do some more investigation when I'm home later today.

First thing for me to check is if the internet connection is going down or not. I forgot about the LEDs because I turned them off years ago - so i'll turn those back on :-)

I could maybe connect a laptop via ethernet to the second port on the main Deco and see it the internet connection is consistant there.

But the WIFI is definitely dropping out when my Deco is connected to the ONT. With the ethernet cable between the ONT and the main Deco removed then I am able to connect to the Deco Network from my android phone via WIFI and check the status of my mesh network and see all my clients (around 15 active) and change settings as normal on the App.

If I plug the ethernet from the ONT in then the connection to the internet is established OK and it will work for a minute or so but then get into a loop of dropping and re-establishing WIFI i.e. I intermittently lose the ability to connect to the Deco Network via the App on my Phone.

The WIFI connection comes back on its own within a few seconds but I can see that the mesh connection to my 3 satallite Decos has been interupted as it takes a few moments for them to come back online. It also takes a few moments for all the clients on my network to come back online - so I'm sure the WIFI has dropped for everything.

I'm not sure if the SSID remains visible - hard to tell because the WIFI comes back online so quickly and the "available networks" screen on my phone takes a while to refresh.

To me the whole thing seems more like the Deco is crashing rather that a network droppout.

Everything back to normal now i've gone back to using my BT smart hub router and connecting to that via a Dynamic IP connection - but I'd really like to not use the smart hub if I can.

EDIT: I have already tried the google DNS addresses and I have swapped my main Deco for one of the satalites and the same issue remained.

I'm planning on changing the WIFI password and powering off my satalite Decos with the aim to get a simple network setup consisting of just the ONT, 1x Deco and my phone and see if that works.

Is there any avanced diagnostics or logs I can view?

Turns out the issue is with the ipv6 connection 

The constant loosing WiFi problem goes away when I disable ipv6.

I found a combination of settings that allow me to connect to ipv6 but ipv6 test sites say it doesnt work anyway so might as well just turn it off.

Speed test results still 10% lower when than when I use my smart hub 2 though :-(

Is that expected?

  @klipse 

Thanks so much for your reply - yes - those are the settings that I ended up with.

I think it was the "get IP address" specifically that I had to alter to "non address" like you to stop the constant drop outs.

But actual ipv6 tests like on https://test-ipv6.com/ still fail though - but they also fail through my smart hub 2 as well so I think there might be a legitimate BT internet service issue there in the background that I will chase up with them.

EDIT: these settings do actually work for ipv6 - I'm getting 10/10 on https://test-ipv6.com now.