I'm highly suspect it is your router to blame.

Casting require a lot of bandwidth. I'm not sure which model of mi wireless router you have. It could be a MediaTek based SoC solution which isn't particularly good at wireless.

After 5-6 hrs of high load it's not surprising that the 5Ghz wifi chip might become overheated and unstable. After all it's just a consumer grade device and it's not very often that it has to handle that much data at that long. That's could be the reason you have to reset the router.

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.....or it could just be a bug in router's wireless driver