@Mikesb I had the exact same problem with my non-pro XE75 units. My only solution was to swap the ethernet cables from the fibre modem to a different socket on my main Deco and for whatever reason my connection started working again. Eventually my satellite units andsome of my clients also stopped working via their usual ethernet ports so I had to swap all my cables around to "fix" the issue. I ended up RMA'ing the whole system and the replacement seems to still be working a few months on.

  @Mikesb when you say a BT broadband hub do you mean the fibre connection coming directly into the property or do you mean the BT internet router supplying the ISP wifi? If you have an ISP router and both are operating as a router that could be your issue, either set the BT hub to act as router (but disable wifi and set Deco to AP mode) or completely remove the hub and set the Deco as a router and connect it directly to the fibre box. That is assuming you have FTTP connection though.

Can you tell us more about how your network is set up?

  @Mikesb just to be 100% sure, is the BT hub set up in "modem only" mode? And if not, have you considered setting the BT hub as the router (still with wifi disabled) and setting the Deco mesh up in AP mode?