Hi @Anonymous User
Have you cast NOW to your TV before or is this the first time your are trying it ?
To cast with your iPhone you need something like a Chromecast connected to your TV or an Android Smart TV that has the inbuilt Chromecast feature.
If your iPhone runs on iOS13 or above and have an Airplay 2 compatible device on or connected to your TV this will also work.
You can also cast from your iPhone to a NOW box or NOW Stick but I have never really seen the point of this myself (perhaps I am more old school and prefer a streaming box or stick with a physical remote).