There are 2 versions of the Dronelink app, Dronelink (for DJI MSDKv4) and Dronelink DJI (DJI MSDKv5).
If you are using multiple drones with a single remote controller, and those drones require a different Dronelink app, you will also be required to update the RC firmware. Otherwise the drone will not connect in the Dronelink app.
Force close the Dronelink apps, and open the DJI app. Then select Connection Guide, and select the drone you are wanting to connect to.
The DJI app will show the following message if it detects the incorrect firmware.
Select Continue to update the firmware.