Xiaomi
Block Apps from Using Mobile Data or Wi-Fi auf Xiaomi
On Xiaomi phones, MIUI assigns every installed app both a Wi-Fi and a mobile data permission by default. If an app can reach the internet through either connection, it will pick whichever is available first, which is why background traffic can hit your mobile plan even while Wi-Fi is active.
This behavior comes from the per-app network controls inside Data usage. Each app listed there has separate switches for Wi-Fi and for mobile data, so you can turn either one off without affecting the other.
To change this, tap on Settings ➔ Connection & sharing ➔ Data usage ➔ choose the app you want to restrict, then turn Mobile data or Wi-Fi on or off as needed.
On dual SIM devices, mobile data can be limited to SIM 1 or SIM 2 individually, which is useful if one SIM has a smaller data plan.
Turning off mobile data for an app forces it to work only on Wi-Fi, useful for large downloads or background sync tools. Turning off Wi-Fi for an app forces it onto mobile data only, which can help route traffic through a VPN or hotspot that runs over mobile networks.
If an app stops loading content entirely after this change, check both switches again, since disabling both connection types blocks internet access completely.