Alexa not controlling Plex on LG 43UR91006LA

Server Version#: 1.32.6.7557 running on Synology NAS
Player Version#: 5.70.0
WebOS Version#: 8.1.0-4806 (number 1-nairobi)
Recently got the LG 43UR91006LA Smart TV and have installed Plex. It works flawlessly controlling it via the supplied LG remote.
Using Alexa I say "Alexa: Tell Plex to play … " and it replies “OK, enjoy … on LG 43UR91006LA” and Plex on the TV does nothing.
While playing something with Plex I say “Alexa: Tell Plex to pause/play” or whatever and it does nothing.
All software is up to date.
Echo and TV are on same WiFi network.
Plex skill is installed on Alexa.
TV is shown in Authorised Devices.
Alexa obviously recognises Plex on the TV from its reply when asking it to play something (see above) but it will not control the Plex Player.
I’ve Googled everything I can think of and checked all settings, uninstalled and re-installed everything, disabled skill and re-enabled it, turned everything off and on again, etc.
Alexa plays music from the Plex server perfectly on all my Echo devices so it obviously works with the Plex server, it’s just on this TV that it has problems.
Any suggestions would be gratefully received.
Anyone else have the same issues?
TIA

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