1. The data connection is not a mobile data network using the main card. If the primary card does not open the mobile data network, then the secondary card will use the mobile data network, resulting in traffic charges.
2. The secondary card is not set or the traffic packet is not enabled. Even if the network card is set, the traffic will be deducted from the main card. You can go to the phone settings, check which is the default network card, and make sure that the secondary card has started the traffic packet.
3. The location of the secondary card is not within the coverage of the main card's 4G network, such as basements, elevators and other places, and the signal is weak, which leads to the failure of the main card's mobile data network to cover the secondary card, resulting in traffic charges.
4. The secondary card is judged as a virtual card by the mobile phone. Although the above forms of network cards appear, there is actually no physical card, so you can't enjoy the preferential policies of network cards, such as free calls. You can enter the phone settings to check whether the default network card is a physical card, and confirm whether the secondary card has the Internet access function turned on.
If none of the above is true, please consult the customer service of the mobile phone operator for more accurate information.