I’m not sure this is the correct steps for an Onn box, but try going to the Settings, tap the “Connected Devices” option, then select “Cast Screen”. Tap the “Stop” button to turn off screen mirroring on your Android device.
OR, Maybe try Settings, then System, and then Cast.
I’m gonna give you a link, where the same installation was successful… maybe you can get a tip from the guy in this thread.
https://troypointinsider.com/t/xfinity-stream-on-walmart-4k-streaming-box/91298