Safe mode is a great idea in this case. You'll be able to see if the notifications work with just what came on the device, using the factory default settings. This will help you identify app or setting conflicts. If it all works as intended, then it's just a matter of identifying the offending app.
If this doesn't isolate your issue, consider the possibility that the update file that you downloaded may have been corrupted. This is possible if your phone doesn't maintain a good and constant contact with the network while downloading the update. If that happened, the FDR would make no difference because you wouldn't be replacing the OS itself.
The fix could be to reload the update. I would have expected the Samsung folks to do this. Download and install Samsung's Smart Switch for your PC. Here's a YouTube video that explains how to use Smart Switch. It's not my video. This for a Galaxy S6 Edge, but the process is the same. The Smart Switch application has been updated (this video is almost two years old), so it looks a little bit different than in the video. The part that you need is almost the same. In the video, the tab you want is labelled "Software Update and Initialization". Now it's called DEVICE INITIALIZATION.
https://www.youtube.com/watch?v=y8AZyalTf5Y
1. Connect your phone and hit the MORE button on the upper right corner of the Smart Switch window.
2. Click on EMERGENCY SOFTWARE RECOVERY AND INITIALIZATION.
3. Click on DEVICE INITIALIZATION.
4. Click OK and follow the prompts.
The software will grab the latest OS for your phone, put the phone into the correct maintenance mode, and install the OS with the latest update. This should leave your phone as though you took it from the box with Nougat preinstalled.