My iPhone 12 no longer finds Airplay when I try to mirror

Activation shows failed what does that mean???

The activation process doesn’t seems to be working at this time for older models Apple TV. You can fix the activation, as described here: discussions.apple.com/thread/253315520.

if does not work over the internet so all must be on the same lan network

some routers keep wired ethernet, 2.4ghz and 5ghz as separate networks, for devices connected to separate one the router need to be set to bridge them for the devices to communicate

people who experience network issues often resolve them by powering off their devices and wifi router for a bit and then turn them on again so they "reboot" so to speak.

AirPlay to what? Do you have the Apple TV box? If so, which model? Identify your Apple TV model - Apple Support