I sent my aircraft away to DJI for a gimble repair. When it returned, the controller does not connect to the aircraft anymore. Not sure if the two events are related. I had no issues with the connection before that.

The controller just shows 'Connecting...' and that's it nothing happens. I connected the aircraft to the computer and updated everything using DJI assistant 2. But that doesn't update anything on the controller. I cannot update the controller through the phone either. The DJI app says 01.04.0100 for remote controller and there is no update option. I tried connecting the controller to DJI assistant 2, but it doesn't recognise it. Talked to DJI support and they think the controller should be detected by DJI assistant 2 and I should be able to update the firmware. But, that is clearly not the case.

The current remote controller version is 01.04.0100 as identified through the DJI GO app.
The current aircraft firmware version is V01.04.0300 as identified through dji assistant2.
The DJI GO app is 4.2.6
The communication link on the aircraft is set to RC.

I suspect the communication issue might be due to different controller and aircraft firmware version. But, I simply cannot update the controller.

Any suggestions are appreciated.

If you sent just the Mavic in for repair, then DJI would have linked it to their controller for testing.
You just need to re-link the controller and then see what updates it needs.

Yup what @Harbourside said just go into settings and relink your controller.  Should link no issues.

What Harbourside  and burnham187 said. Looks like you just need to relink the controller to Mavic in the GO 4 APP.

as the guys have said above - relink your controller - either through the app or without the app, both are easy. Check also that they did not maybe send you a new replacement drone. if this is possibly the case then maybe you need to do the authentication/registration process again.

Lol, they sent you somebody else's broken drone. Classic

Hi there, I'm afraid that linking RC might fail if the aircraft and RC firmware are not matched. Please try to hold the three bars on the right upper concern of APP for a certain time until a window pops up, select firmware version to see whether the update is successful. Then link RC again to see whether it helps.

Ahaaan. I wasn't aware of this re-linking thing. And I am surprised DJI online help didn't recommend this to me. I tried it out and it worked. Once linked, I was able to update both drone and controller through the DJI 4 app. Thanks a lot guys!

Excellent glad you were able to link it.