It does not automatically fall back to LTE backhaul. Tries to reestablish with your Wi-Fi backhaul first. If that fails, you have to manually change backhaul to LTE.
So, any insight in “Project Sprint Dark Fiber”?
https://www.ct.gov/csc/lib/csc/ems/hamden/wintergreenave/sprint/em-sprint-062-191009_responseandacknowledgement_wintergreenave.pdf