Hello,
I have the same problem with my MI5, after update from xiaomi.eu 8.5.2.0 to 8.5.3.0 I can't activate WiFi. When I put the WiFi switch to On, this return instantly to Off!
UPDATE: In my case, it was a software problem. I downgraded to 8.5.2.0 >> Wi-Fi OK; after that I updated again to...