Why won’t my iPhone stay connected to wifi?
I connect to any wifi and if I’m dormant on my Iphone for >1 min, it disconnects from wifi
Settings-General-Reset and reset network settings.
If the problem persists, check and see if it happens on all networks you use rather than a single network.
If it happens on all networks, do a restore via iTunes and if that does not fix the problem make an appointment at the Genius Bar.
If it still happens on a single network, see if there is a firmware update for that router.