This is down to Google's design configuration, not because of the projector.
Google has a mechanism that when using Google cast, the device will periodically connect to a Google server in background to synchronize and verify.
When using hot spot mode, it cannot connect to a Google server to synchronize and verify so, when the verification has expired, the casting cannot be processed.
In order to fix the problem, it is suggested to have the projector and android handset connect to the same router that can also access the internet. After connecting both devices, please restart the projector and keep it connected to internet.
Please use Google Home or Chrome browser cast.
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