I have this set up involving a Mi Box 3 that has no access to an internet connection.

I know that Chromecast Mirroring requires both devices to be on the same network, so I used another phone as a "no internet" hotspot to act as a bridge.

However, the Mi Box 3 refused to pop up as a screen mirroring option unless I switch on mobile data on the other device. I can turn off mobile data once screen mirroring has been enabled.

Does anyone know why this is happening? Am I doing something incorrectly? Thanks.