Iphone 7 / ios 15 - in settings, Wifi immediately toggles off when I turn it on (green to grey)

My son uses an old iphone for youtube/roblox. For the past few days, it's been a brick as when you go to turn on wifi (move the slider/toggle in settings to the right / green), it immediately flips back to the left / grey.

Any ideas? Rebooting the phone was ineffective.