Damn, Nostrica flight gets back after their office hours close at O'Hare. And departs before they're open!
This is the shortcut: https://www.cbp.gov/travel/trusted-traveler-programs/global-entry/enrollment-arrival
Look into Enrollment Upon Arrival if it’s not approved beforehand.
Discussion
Ah, man. Tough break. As others said, it rarely takes six months for an appointment regardless. Hopefully you get lucky!