Xiaomi
Set Default SIM for Mobile Data auf Xiaomi
On dual SIM Xiaomi phones, MIUI only routes mobile data through one SIM card at a time, even if both SIM slots have active plans. If no default is set, or the wrong SIM is picked as default, apps that need data - maps, messengers, browsers - may fail to load or fall back to Wi-Fi only, while the SIM you intended to use for data sits idle.
This setting lives under Tap on Settings ➔ Tap on SIM cards & mobile networks ➔ Choose SIM 1 or SIM 2 as default Data card. Choosing a SIM here tells MIUI to always use that card for mobile data, regardless of which SIM you use for calls or texts.
Effects: once set, mobile data will not switch automatically to the other SIM, even if that SIM has a stronger signal or more data left on the plan. If you swap SIM cards physically or cancel a plan, you need to update this setting manually, otherwise data may stop working entirely.
This fix is not the right one if data actually works but is slow or drops intermittently - that points to a signal or network mode issue rather than the default SIM setting. It also will not help if a SIM has no active data plan at all.