There are two more settings to examine for decoupling the cellular and Wi-Fi networks. Samsung has tried to intelligently couple the two for improved user experience with their Note 7. That aside, take a look at these settings and turn them OFF:
Settings > Wi-Fi > 3 dots > Advanced > turns OFF all sliders (specially Smart switch Network)
on this screen set : Keep Wi-Fi on during sleep > Always
Mobile Hotspot and Tethering > turn OFF all sliders
If these work for you, then you can go back turn ON some of the sliders for services you wish to retain. Do share your findings.