Take it you don’t have any other active memberships with NOW to give it a test?
Its always worth trying your luck to test by purchasing and if it doesn’t work you could jump onto live chat to complain. They might be able to give you a refund.
But when Now give you the three month’s Mobile Pass you get with a Day Pass, you won’t be able to cast that.
With regards to the purchase of the NOW Day Sports Pass.
If Air Play was supported when using the NOW App on the iPhone, then i would expect it to be mentioned on one of the NOW Help Articles which i haven't come across.
You could previously cast from an iPhone to a NOW Box / NOW Stick (no idea if it's still broken or fixed), but i never understood why anybody would do this having a NOW Box or NOW Stick already connected to your television apart from using the iPhone has a remote controller if you have lost or broken the physical NOW remote.
Also double check you don't already own a suitable playback device from this list in the link below.