Update September 3 =Looks like disabling Wake on LAN in the BIOS did it. I still don't understand why it wasn't a problem with cable internet, but became so with fiber. wdaren
Just in the last 2 days, suddenly my computer will turn itself on about 10 minutes after I shut it off.  When I turn it off again, it will be off, and then turn on again ten minutes later. The only way to keep it to stay off, is to hold the button until it shuts off, or unplug the computer.  I have made no changes to the computer, but I did have ATT install their fiber gateway. But they did nothing to the desktop itself.  I have run DISM, and the Dell scan. 
XPS 8930, turns on after being off