https://docs.oracle.com/database/121/DGBKR/cli.htm#DGBKR3431

6.12 Scenario 11: Reinstating a Failed Primary Database
To reinstate the failed primary database, start it to the mounted state. Then run DGMGRL, connect to the new primary database and reinstate the old primary database.
Step 1   Restart the Old Primary Database.% dgmgrl connect sysdg
Password: password
Connected as SYSDG.
DGMGRL> startup mount;
ORACLE instance started.
Database mounted.

Step 2   Reinstate the old primary database.% dgmgrl connect sysdg
Password: password
Connected as SYSDG.
DGMGRL> REINSTATE DATABASE 'PROD_SC';
Reinstating database "PROD_SC", please wait...
Reinstatement of database "PROD_SC" succeeded
 
Database dismounted.
ORACLE instance shut down.
Operation requires startup of instance "PROD_SC1" on database "PROD_SC"
Starting instance "PROD_SC1"...
ORACLE instance started.
Database mounted.
Continuing to reinstate database "PROD_SC" ...
Reinstatement of database "PROD_SC" succeeded


After the primary has been reinstated, issue the SHOW CONFIGURATION and SHOW DATABASE commands to confirm that the old primary has been successfully reinstated.
Step 3   Show the Configuration and Databases.DGMGRL> SHOW CONFIGURATION
 
Configuration - DRSolution
 
  Protection Mode: MaxAvailability
  Databases:
    PROD_PC  - Primary database
      PROD_SC  - Physical standby database
 
Fast-Start Failover: DISABLED
 
Configuration Status:
SUCCESS

DGMGRL> SHOW DATABASE 'PROD_PC';
 
Database - PROD_PC
 
  Role:            PRIMARY
  Intended State:  TRANSPORT-ON
  Instance(s):
    PROD_PC1
 
Database Status:
SUCCESS
 
DGMGRL> SHOW DATABASE 'PROD_SC'
 
Database - 'PROD_SC'
 
  Role: PHYSICAL STANDBY
  Intended State: APPLY-ON
  Transport Lag: 0 seconds (computed 1 second ago)
  Apply Lag: 0 seconds (computed 1 second ago)
  Apply Rate: 0 Byte/s
  Real Time Query: OFF
  Instance(s):
    PROD_SC1
 
Database Status:
SUCCESS

 