Recently they (official app), allowed sharing the screen to a chromecast device/tv...so instead of mirroring try streaming it through a chromecast connection. I don't believe the screen mirroring connection works with the modded or unmodded app.
As far as connection to chromecast device might need to set up a hotspot on a separate device, then connect main phone (with Xfinity app) and chromecast device to the hotspot (so they're on the same network).
The above may work from your main phone (setup hotspot, connect chromecast and stream Xfinity) without the need for another phone to make hotspot network but i haven't tested it.