As you can see yourself, you have a device specific problem.
The alarmmanager extension offers a lot of options. My recommendation is to start as simple as possible and use the SimpleNotification project as a start. Please see the documentation. Use the apk file simple apk from the download folder. This example requires only PostNotification permission.
Get that example running on your Redmi first before continuing and try these device specific adjustments manually in the settings
Taifun