Plex supports controlling the Plex Android TV app from other Plex clients, such as the Plex iOS app.
See Plex Documentation → Casting, Flinging, & Remote Control.
There is no support for controlling the Plex Android TV client from non-Plex applications.
You might be able to launch, etc the Plex app using Roomie, but it would basically be substituting for the Shield remote.
I’ve not seen a way to directly control the Plex Android TV app from a 3rd party application.
Plex Media Player is a discontinued Windows/Mac application.
The Shield runs the Plex Android TV app.
There is no “old player” in the Plex Android TV client. It went away quite some time ago and was unrelated to the Plex clients on the AppleTV and Roku (the ATV & Roku clients are completely separate from the AndroidTV client).
Please verify the Plex client installed on the Shield. The current release is 9.12.2. There is no 9.1.1 release mentioned in the announcement thread. 9.2.0 was released in May 2022.
Does the Plex app on the Shield play media correctly when using the Nvidia Shield remote?
When playing media using the Plex app, what is listed in Plex Dashboard → Now Playing? Can you attach a screenshot?
