How to downgrade iOS 11 beta to iOS 10 without PC?
It's impossible without a PC or a Mac to initiate the wipe and also host the reinstall image download and USB control during the downgrade. 

In addition to the "computer" - you need an Apple tool like iTunes (Mac or PC) or Xcode or Apple Configurator (or a third party one like Ground Control) to reload the older OS, since there is no OTA bundle for downgrading.