You already did some worthwhile troubleshooting. I will suggest some other things to try: If still not working, again run Diagnostics via boot with F12, and run extended tests. Make note of any error messages and error codes. Did you ever perform a clean install on that laptop? If so, did you install the required chipset drivers from Dell Support site for XPS 15 9570?  
I ran Dell Update on my dad's XPS 15 9570 and when they finished, I restarted and I noticed that it wasn't connecting to WiFi so I tried Ethernet and it didn't work then. I looked at the adapters in Device Manager and it showed "Windows is still setting up the class configuration for this device. (Code 56)". I disabled and enabled, it didn't work. I ran the troubleshooter in settings, it didn't work. I ran SupportAssist and got error codes "D0605" and "D0702". I ran the F12 Diagnostics and it said "Critical Error Message One or More Error were Found". The battery in this laptop is almost reaching the end of its usable life so I don't know if that could be causing it. I need this fixed asap. I am out of warranty so I don't plan on calling Dell until I need to.
XPS 15 9570, WiFi and Ethernet not working