IPad Air won't play any videos now
First make a backup using iTunes or iCloud. As Apple Support suggest their first solution would be do a Network settings reset. I think that what you said you have done already . If you have already done that, they said to reset all settings. That would probably solve your problem. If that ain't help either just do a Erase all content and settings reset. If neither of them work properly, you have to take that into Apple support center.  Before do any of these reset, I highly recommend to backup device.