Samsung devices use SIM card 1 for mobile data use by default. SIM card 2 can also be selected in the SIM card management. However, some Samsung devices only support fast LTE on SIM card 1.
Note: The default SIM card for mobile data can also be changed via the status bar.
- Open the Settings
- Tap on Connections
- Tap on SIM card manager
- Tap on Mobile data
- Choose between SIM 1, SIM 2 and Off