@David-TP I've not tried the guest network as I'd like users on the main LAN to be able to cast YouTube to the TV, hence do not want to be on a separate network.  I have found a work-around by setting up an old Bekin (sorry!) wifi extender plug configred onto the DECO Wifi, configuring the Belkin's extension Wifi SSID with a diffrent network name to the DECO Mesh and then connecting the Samsung TV to it.  A clunky implementation but it works!

Regarding the printer - I believe it was set up previously with a fixed IP address, though I gave it a factory reset once I found it was not connecting to the new DECO mesh (When I installed the DECO mesh, i shut down my old WiFi network and configured the DECO with the same SSID and password as the old network - far simpler than having to go around 20 or 30 devices to reconfigure their WiFi!) 

The printer doesn't have a display or any form of error message.

The orignial Epson installation software allowed me to connect to the printer via USB and configure a WiFi connection, but that software is no longer operational under my current version of Windows, & the latest Epson config seems to rely on a WiFi connecction (or simply says "press the WPS button on your router and wait for the printer to connect")