I want to link my iPhone to my Roku TV.

You can also connect your device to your TV using a cable if your TV has an HDMI port. This method doesn't require the internet. But, not everyone likes dealing with cables.

That's why I often use the wireless method instead, through mirroring apps. DoCast is a good choice for that https://apps.apple.com/us/app/docast-cast-and-mirror-to-tv/id6446948344

If your Roku TV supports AirPlay, it's simple. Just connect your TV and iPhone to the same Wi-Fi, open the Control Center on your iPhone, and tap on Screen Mirroring. Your Roku should show up in the device list.

If your Roku doesn't have AirPlay, you can use mirroring apps from the App Store, like DoCast, Replica, or AirBeam. These apps let you share your screen and also stream photos or videos, which is handy. I use them to show presentations from my iPad or iPhone when I need to.