@Tiz 

Usually, connecting Smart TV by Ethernet to Deco unit fixes such issues. I would suggest three troubleshooting steps: 

1. Verify that TV uses Ethernet link while connected by Ethernet cable.

I briefly owned TV where I had to manually disable WiFi interface in configuration settings to make it use Ethernet.

You can find how Smart TV is connected from Deco app.

2. With TV connected by Ethernet to Satellite Deco, verify that Satellite Deco has good signal connection to Main Deco.

If Satellite Deco has weak signal connection to Main Deco, that will negatively impact all devices connected to Satellite Deco.

You can find that info in Deco app.

3. Lastly, if after steps 1 and 2 the problem is still there, and as long as you have your ISP provided router with Main Deco connected to it, you can configure Deco mesh to run in Access Point mode.

In Access Point mode, all bandwidth management and content filtering features of Deco mesh will be disabled. Some of them might be impacting quality of streaming. How to set up Deco to work in Access Point mode

  @Alexandre. 

Thanks for the quick reply

1. Verify that TV uses Ethernet link while connected by Ethernet cable.

I reconnected the TV and it and the Deco app confirmed a wired connection

2. With TV connected by Ethernet to Satellite Deco, verify that Satellite Deco has good signal connection to Main Deco.

The TV is connected to the Main Deco rather than a Satellite Deco

3. Lastly, if after steps 1 and 2 the problem is still there, and as long as you have your ISP provided router with Main Deco connected to it, you can configure Deco mesh to run in Access Point mode.

This is what I've been doing and it works in AP mode, however, I would like to use all the features so I may have to return it and try another solution.

  @Tiz 

One more thing to check. Bring Satellite Deco to where Samsung TV is, connect TV to Satellite Deco with Ethernet cable. If that works, you might have IPTV enabled for the second port of Main Deco: https://www.tp-link.com/en/support/faq/2421/

  @Alexandre. 

Thanks again, I will check that, but I know there is a page on the Deco app when setting up a network that specifically asks about internet connection type and has an option to switch on/off VLAN/IPTV.  I left this switched off so I'm not hopeful this will fix the problem.