First, you will need to determine which client is making the request. Then you can chase down applications on just that client. In your query log, are you seeing requests from individual clients or are all requests shown as coming from your router IP?
xiaomi.net indicates a xiaomi device - perhaps a router, security camera or the like.
You can also tail your pihole.log, which will show you more detail on the query. The request, any aliases (CNAMES) it leads to, and the eventual returned domain IP.
I know from the IP that it is the mobile of my wife. As she heavly cheated on me in the past, I fear this can be a hint that she does it again. So it would be helpfull for me to find out which app this is, or if this normal behaviour of the app or chat messages from her.
The device that's requesting those, is located at 192.168.188.51 as for the domain itself, it's a telemetry domain for xiaomi devices.
It can be anything that's manufactured by xiaomi.