@Mwburton63 iOS doesn't support Chromecast natively. There's a little bit of a "jerry-rigging" you'll need to do but it's not too tough an endeavour. Here's a guide from Android Authority that might help. Bear in mind you can probably skip the steps to install the Chromecast if it's built into your TV:
Are you using a newer iPhone SE, or the 'old' one (1st generation from 2016)? If newer, what version of iOS are you running and what version of the TELUS TV+ app are you on? First troubleshooting step should be that both are up-to date.
When you say 'cast' are you trying to use Airplay or Chromecast? You should be able select either from the player in the TELUS TV+ app depending on what receiver device you want, see the screenshot below. You won't be able to Chromecast to an Apple TV, and you wouldn't be able to Airplay to a Chromecast with Google TV.
Finally, there IS a TELUS TV+ app for Chromecast with Google TV in the Google Play Store, and there is also a listing for Apple TV in the Apple App Store. Casting is a great solution when there is no app available, but I find reliability of a native app superior to Airplay and Chromecast.