Realme
Set Default SIM Card for Mobile Data auf Realme
On a dual SIM realme phone, mobile data always runs through one SIM card at a time, even if both slots are active. Realme UI keeps a single default for internet access, and it does not switch this on its own when you insert a new SIM, change plans, or start using one card mainly for calls.
If the wrong SIM stays selected, apps that need mobile data will fail to load or fall back to Wi-Fi only, since the connected line has no active data plan or a weak signal.
To fix this, go to Settings ➔ SIM card & mobile data ➔ Internet and choose SIM1 or SIM2 as the default for data.
After switching, the phone searches for the new network and registers the SIM for data again, which can take a few seconds. If the connection still does not work afterward, check whether the correct APN is set for that SIM, since some carriers require you to add it manually once.
Switching the default SIM for internet does not change which SIM handles calls or texts. Those can stay assigned to the other card, so you can keep one SIM for voice and the other for data without extra setup.