Huawei
Set Default SIM for Mobile Data auf Huawei
When two SIM cards are active on a Huawei phone, EMUI needs a clear rule for which one carries mobile data traffic. Without that rule, data usage can jump between carriers, causing billing surprises or inconsistent speeds, especially if one SIM has a weaker data plan or no data plan at all.
This setting only affects mobile data. Calls and texts keep their own separate SIM assignment and are not changed by this step.
Where to set the default data SIM
Go to Open the settings ➔ Tap on Mobile network ➔ Tap on SIM management ➔ Choose the default-SIM for mobile data. Selecting a SIM here locks mobile data to that card until you change it again.
If the option to pick a SIM is missing or greyed out, check that both SIM slots are recognized and that mobile data is switched on in general. A single active SIM will not show this choice, since there is nothing to assign it against.
Switch the default SIM back if you swap a physical card, cancel a data plan, or travel and want to avoid roaming charges on one of the two numbers.