@Profly 

Im guessing when you have the Deco connected and in Router mode, you are not going straight from the Deco to the Internet?   Basically are you still using the ISP router cabled into the Deco?

ISP Router..  >   DECO  >   Clients..

If so it could be a NAT issue, you might need to setup some port forwarding on the Deco to allow it to pass the traffic straight to the internet.   

  @Philbert 

The ISP router is not included in the setup. The first X20 is configured in router mode, and connects the internet via Ethernet port 1 to the FTTH modem.

The Hue bridge connects to same X20 unit using port 2. I have tried changing the cable and swapping the ports to no success.

Reverting the the original setup prior to buying the Deco units, and everything works correctly.

If I include the ISP router into the setup, as you mentioned in your post, the Hue bridge also functions correctly. However, the intention was to have a clean setup using just the X20 units.

  @David-TP 

I did try several cables with no success.

Re the IP address. I am unable to see the bridge in the Deco app, and likewise via the Hue app, to have access to changing the IP for the bridge.

Philbert mentioned port forwarding, but again, as the bridge is not listing as a client, I have no way (that I understand) to configure anything.

Thanks for your help.

  @David-TP 

@Philbert

Figured it out!

During the setup for the X20 in router mode, the default for the app was forcing me to have values set against the IPTV VLAN.

I was unaware that this results in the remaining free physical Ethernet port being dedicated to that service.

Once I went back into the VLAN menu in Advanced settings, I was able to deactivate IPTV. The bridge straight away made the proper connections, showing as a client, and I have full functionality in the Hue App.

Thanks again.