Can you watch movies from an external USB flash drive on an iOS device?
You need to pick an external store device that comes with an app to play content on the device.  


  Stream videos to up to 3 devices—at the same time
  
  SanDisk Connect Wireless Stick is a great entertainer, too. You can
  stream music or videos1 to three smartphones, tablets or computers as
  long as they’re somewhere in the vicinity—the back seat, the next tent
  or the other end of the conference table.


https://www.sandisk.com/home/mobile-device-storage/connect-wireless-stick