I finally got my service message to reset. Here's how:
Put the key to position 2.
Use arrow keys so it shows how many miles you've driven (part of the first trip series of options).
Hit the phone key and hold it down.
Immediately hit the OK button and hold it down as well.
In 10 seconds up comes a menu to follow to register that you've done the oil change/service which resets ASSYST Plus to 14,900 miles.
This video explains it:
Based on the written guidance I was doing a bunch of things similar to that, but not quite it.