What a bloody nightmare and exercise in frustration.I've viewed EVERY YouTube and DJI video on setting up the Ronin S.  None of them addressed a Nikon D850.  The D850 setup shouldn't be that different from a Canon or Sony, but it didn't work.  
I could not get the Ronin S to even BEGIN to balance.

Last eve, I did the firmware update and hoped that would solve something.

NADA.
After 3-5 hours of trying to get this bloody thing to begin balancing, I gave up and requested an RMA from DJI.
I wish it would have worked.  I could have used it!

More info would help.  

Can you not get it balanced with the motors off or when the Ronin is powered up?

I'm sorry to read on the mishap, Grtday. As you had sent the unit in, the local team will help when they receive it. Hope you can get the unit back soon, and if there is any assistance needed during the repair, please feel free to let us know.​

FlySD Posted at 2018-10-12 07:47
More info would help.  

Can you not get it balanced with the motors off or when the Ronin is powered up?

Everything I read said to do it unpowered and then turn the power on.
Still nothing from DJI

DJI Diana Posted at 2018-10-12 12:03
I'm sorry to read on the mishap, Grtday. As you had sent the unit in, the local team will help when they receive it. Hope you can get the unit back soon, and if there is any assistance needed during the repair, please feel free to let us know.​

I sent in a request to return it for credit.  
I have not heard back from DJI re: the return.
Who should I talk with?
Thanks Diana!

Grtday Posted at 2018-10-12 12:17
I sent in a request to return it for credit.  
I have not heard back from DJI re: the return.
Who should I talk with?

Hi Grtday, sorry for the troubles with Ronin-S, I managed to check your email via your Forum account and our support team has replied the email.
Before sending in, I would like to make a brief troubleshooting, upon checking your serial number, your Ronin-S hasn't been activated, therefore, it will not stabilize itself even though you have physically balanced it before you power on, could you please refer to the screenshot below to activate the device first to try again?

Hi, I am not able to setup the Ronin S to my Nikon D850. As soon as I connect the cable to the Camera, my view finder on the Camera goes blank and I am not able to record or control the camera from the Ronin

vallala Posted at 7-25 18:03
Hi, I am not able to setup the Ronin S to my Nikon D850. As soon as I connect the cable to the Camera, my view finder on the Camera goes blank and I am not able to record or control the camera from the Ronin

Which cable are you using? Please ensure the firmware of Ronin-S and camera are the latest version. Please check the details of how to setup here: https://www.dji.com/support/compatibility/ronin-s/d850