I was having a heck of at time getting my smart remote to connect to assistant 2 so I could upgrade the firmware.  It would not connect no matter what I did.  In my research I found out what was causing my issue and possibly it might be the issue of many others.  
I resolved my issue from someone that responded and told me that I had to ENABLE USB DEBUGGING in DEVELOPER OPTIONS.  Yet, when I tried to do this; I quickly found out that Developer Options on my smart remote was missing and not viewable.  Furthere research from another forum poster led me to this solution:

To get Developer Options unlocked... Go to settings and scroll down to About Remote Controller... Tap it. Now scroll down to System Version and tap it (7 times) to unlock Developer Options. Go back and it will be in your list.  Yes, like knocking on a door-tap seven times!  Odd, but it worked!

You would think that this would be be more common knowledge due to all of the posters I see that is having trouble getting their smart remote to show up in assistant 2.  Took me 4 days to finally find this out!  Hope this helps someone out before they spend days on end like I did!  

Although not related to my drone, I recently could not get Android auto on my brand new Lexus RX 350 to recognize the Waze GPS app. Believe it or not even the Lexus technical representative could not figure it out. I finally did a search and found someone who had an Acura MDX who reported he was able to get his to work by enabling developer options on his smartphone. As soon as I enabled developer options on my smartphone, the Waze app began working in Android auto.

Perhaps, developer options should always be enabled for all apps.

Hello and good day M Stuart K. Thank you for sharing these information and for sharing your experience in connecting your DJI Smart Controller to the DJI Assistant 2. This thread will be a very big help to all our DJI members who has the DJI Smart Controller. Thank you for your valued support.

many thanks for sharing

Thanks a lot. It's always good to know it.

Do you know how work the developer mode in DJI RC Pro?? Thanks !!