Help please.

Ive just received my 3 pack of AC1900 Deco Mesh S7.

I have a BT Openreach SmartHub router, and I have decided to connect my main Deco unit to it via an ethernet cable, and have set the Deco unit to AP mode.  I have connected my phone to the new Deco wirless network - so far so good.

I am getting the following error in the app interimittently ....Make sure that the main deco works well,  Reconnect.  Alternatively, restart the main deco until the LED turns solid green (mine is the version with the white led).   

When I refresh the screen, it says Everything looks good - then the same error again every 20-30 secs.

Is my BT smarthub in some way interfering with the connection?  is there another setting I have missed - I havent changed anything else other than chose Access Point mode

IPTV/VLAN is disabled

Mac Clone is disabled

DDNS is disabled

DHCP server wont load, just spins

Would I be better off removing my BT Smarthub and just connecting the Deco main unit directly to my ONT?