iPhone Auto WiFi connect gone

It took me while to figure this out. My 6+ phone was never staying connected to the WiFi. I finally realized it was shutting off when the phone display “locked” either manually or automatically. It does not automatically reconnect the next time it is “unlocked.”

Anyway, I solved it to my satisfaction by turning off cellular on the high data apps like video players. After all, the phone is on during an entire video play so why worry.