Hey @Vincent_Kennedy,
Disabling ambient mode or enabling stay awake in dev mode is not something directly related to Roon — it is Chromecast specific. Searching online about this it seems to affect other streaming apps like Twitch or Youtube.
Disabling ambient mode is explained below, in this Chromecast Help page:
https://support.google.com/chromecast/answer/6080931?hl=en
Finally, it would be dev mode for the device. It should be found in settings - system - developer options - stay awake. The instructions here might be of help: