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.

Android 10 (One UI 2.0)
Step 1:  Open the  Settings
Step 2:  Tap on  Connections
Step 3:  Tap on  SIM card manager
Step 4:  Tap on  Mobile data
Step 5:  Choose between  SIM 1 ,  SIM 2  and  Off
  1. Open the Settings
  2. Tap on Connections
  3. Tap on SIM card manager
  4. Tap on Mobile data
  5. Choose between SIM 1, SIM 2 and Off

Samsung Instructions