Hello triplehjr,
Thanks for reaching out through the Apple Support Communities about mirroring your iPhone SE to your Apple TV 4K.
Are you able to stream video from your iPhone SE to your Apple TV 4K using the steps from the first half of this article? Use AirPlay to stream video or mirror the screen of your iPhone, iPad, or iPod touch
If not, are your iPhone and Apple TV connected to the same Wi-Fi network?
Also, do you use any other devices, such as an iPad? If so, do you experience the same behavior on your other devices?
Hopefully those details will help to further isolate and address your situation.
Take care.