I have signed in on my account and my subscription doesn’t show

I’m not sure, but maybe you will have to go into settings on your iPhone and set up sharing, like I said not sure but give it a go.

The subscription to the Apple TV+ service is valid for your Apple ID on any of your devices. If you have the Apple TV app on Roku, and you are signed in with the same Apple ID, then you should have access to the Apple TV+ channel. Go to the Watch Now tab in the Apple TV app to locate the Apple TV+ channel with its content.