If Spectrum is your ISP and they provide the modem/router, then they have the ability to change settings in the router. It sounds like they might have enabled WiFi security that blocks connected devices from seeing each other.
I don't know if that's a setting that Spectrum will allow you to change on your own. I have Comcast, and they blocked me from the majority of the settings in my modem. I finally got fed up and put the modem in bridge mode and installed my own firewall/router, which I can fully control and Comcast can't touch.
I also have to point out how old your iPhone 8 is, and it might not support the latest updates for screen mirroring/casting. iOS 16.7.8 is the last version update your phone will ever get.
