I was able to fix this problem on my Mi 6x after much troubleshooting. (Not sure if you need a rooted phone for this).
The first answer on this thread worked:
https://forum.xda-developers.com/mi-a1/help/internet-hotspot-8-1-update-t3833265
It was a tethering issue where a setting was enabled restricted tethering.
(Note: In Developer settings I had to have both ‘USB debugging’ and ‘USB debugging (security settings)’ on to be able to alter the setting through an ADB shell. Also note, when testing if the setting has changed, the result has to be ‘0’ (zero), not ‘null’ as I got the first time I tried).
There may also be other options eg changing APN settings under SIM in your phone settings, or changing your phone to allow 3G only (not 4G) etc. Google “tethering” “no internet” (including the quotes), your phone carrier, your phone type etc.
I’ve read that tethering may be becoming more tied down (in some countries?) on Android. So before buying a phone or choosing a mobile carrier, it’s worth researching such tethering restrictions.
