I would've added a comment, but cant since my rep is still too low. So I am asking here:

I'd assume you already did the basic troubleshooting stuff like restarting both devices (pc & phone), disabling and then reenabling USB-Debugging, reinstall Android Studio completely (or just the device mirroring plugin).

But are you able to build and deploy your App onto said phone?

If true, maybe you could try and disable wifi on your device, so that it is forced to use the USB connection to mirror.