By default, the Samsung device automatically selects the best network, but once a network is selected manually, the "Select automatically" function must be reactivated to use the automatic network search.

Android 10 (One UI 2.0)
Step 1:  Open the  Settings
Step 2:  Tap on  Connections
Step 3:  Tap on  Mobile networks
Step 4:  Tap on  Network operators
Step 5:  Tap on  Select automatically
  1. Open the Settings
  2. Tap on Connections
  3. Tap on Mobile networks
  4. Tap on Network operators
  5. Tap on Select automatically

Samsung Instructions