Issues setting up a Chromecast

I bought two Chromecast ultras, when Noel Leemings had them for $92 last week. I thought they would be a good and affordable way to get TV on demand, Lightbox, Netflix and Plex to a couple of TVs.

I have set them both up on wired connections. For the first one, the setup went smoothly, it showed up in the Google Home app as ready to cast, and I am able to cast to it as expected. Working well and no issues whatsoever.

Flushed with success, I moved on to the second one. Again, the setup process went identically and without a hitch, and it shows up in the Google Home app as ready to cast. It is also displaying backdrop art images and the correct time etc (mirrored under devices in the Google Home app) , so it's clearly connected and seeing both the network and the internet. However, at this point things seem to have come adrift. No matter what I do, it continues to show up on Google Home as ready, but no casting app shows it as an available cast target (the first one continues to show up fine).

This is starting to drive me nuts. I have:

done several reboots

done several factory resets

repeated the setup process three times

tried several devices (phone, laptop, tablet)

rebooted the router

changed the name

verified that the firmeare is up to date - it is the same as on the one working OK

All of this has been to no avail. Same result every time, and I am starting to climb the walls. Does any fellow GZer have Chromecast experience and any thoughts about whether there is anything else worth trying?