Stadia was largely touted as a cloud gaming service that’s supposed to work across screens, but with a big asterisk. For streaming games on a TV, you need a Chromecast Ultra unit shipped inside the Founder’s or Premiere Edition packages, at least initially, while the existing Ultras were required to wait for an update. It appears that some Redditors got a new firmware that allows pairing Stadia controllers with those left-out dongles.
In most cases, turning on Preview Program for the Chromecast Ultra from the Home app and rebooting the device helped bump the Cast firmware version to 1.42.180348, which adds the feature. With this build installed, the Ambient mode settings option will show a working Stadia Controller linking code. A user confirmed that the version number matches that of the bundled Chromecast Ultra units, while another user reported having received a newer 1.42.180518 version that also works as intended, though you might have to mind the overheating issue.
It’s worth noting that only a Stadia Controller will work with Google’s 4K dongle for streaming games, even though some third-party controllers can be used with Stadia on Chrome desktop and mobile. Plus, Google is yet to announce its stand on expanding Stadia’s compatibility to non-Ultra Chromecast models, Android TVs, and other TVs with built-in Chromecast.