unable to mirror tv content on apple tv

1. You haven’t specified what you tried to watch (from which content provider, from which app). That may be important for its possibilities, as various content providers have different requirements for AirPlay.
2. Most of the streaming service apps for iPhone also have an app edition for the modern Apple TV box. Then you wouldn’t need AirPlay.
3. You haven’t specified the model Apple TV box. That may be important, as newer models have some streaming capabilities (e.g. AirPlay 2) that older models do not have, and that various content providers may require. Identify your Apple TV model - Apple Support Find your Apple TV model number - Apple Support

To be sure, please ask Spectrum Support if they have minimum requirements for AirPlay from their app to an older Apple TV box, or other requirements (perhaps only on the home network that matches address [and thus IP address] for the plan with them).

apps and sites can choose to block their material to be cast by airplay as this strip their content of it's DRM encryption that protect it from being copied

you can try to airplay something else like some random youtube stuff or just the iPhone normal display

and no, there is no way around it if the app or site active block airplay because of DRM issues