Posting this in case it helps someone else:
I was able to connect to my brand new N950 via Smartthings and all was well until I selected the firmware update. That worked - it's currently 1005.1
But afterwards, the soundbar was not contactable via WiFi any more. I tried many things, including the steps listed above. It would sit there on WiFi Disconnected or WiFi Network (after pressing the Network button) but wasn't connectable or even discoverable as a new device.
Eventually I happened on this approach by playing with the app:
SmartThings|Settings|Update WiFi Information. Once I selected the soundbar, it displayed a dialog which indicated that the bar was "owned" by another account. A spurious error at best, but something to try.
I wasn't able to use the first option offered, which tried to join me to that account (in the process sent a notification to my current instance of SmartThings requesting that the join be allowed!).
However, I WAS able to use the second "Replace the account with your one" option - it involved holding down the soundbar's volume-down (-) button at least twice during the process (a second distinct time when it was at 39%) to approve the change. In the end, it took me two attempts - but eventually it successfully added the soundbar (back) to my current account. All is now good.
I'm paraphrasing the option descriptions above, working from memory as I can't repeat the full process now that all is well.
For those having issues connecting the soundbar to wifi using the SmartThings app, is your phone running Android 9? If so try on a different device with Android 8.1 or lower. I was having the same issue and by accident managed to established a connection using a phone with Android 8.1.