With a nowtv mobile sports pass there is no option to cast to your Chromecast or mirror on a television.
The mobile sports pass will only work on a smartphone screen (not a tablet) and will only provide 5 number Sky Sports channels which are Premier League, Action, Arena, Racing & News.
Have a read of this.
Was you using the Sky Sports Premier League channel ?
Where a mobile sports pass doesn't work on both Sky Sports Main Event & Sky Sports Football channel and a message will pop up on your smartphone that you need to buy a pass to watch.