Oppo
Set Default SIM for Mobile Data auf Oppo
On dual-SIM Oppo phones, calls and mobile data can each be assigned to a different SIM card. If the data SIM was never set explicitly, ColorOS may keep using whichever card was active first, even if that SIM has no data plan or is roaming.
The setting responsible for this lives under mobile data management, separate from the call SIM setting. Changing it does not affect which SIM handles calls or SMS, only which card the phone uses for internet access.
How to change it
Open Settings ➔ SIM card & mobile data ➔ Mobile data ➔ Choose SIM 1/2 and select the card that should handle internet traffic.
After the change, the phone re-searches for a network on the newly selected SIM. This can take a few seconds and briefly interrupts an active data connection.
If mobile data still is not working after switching, check that the selected SIM actually has an active data plan and that mobile data is toggled on for it. A missing plan or a disabled data toggle looks identical to a wrong SIM selection.