You can try something like this to make 'fixed' background to work on iOS in UNA:

.bx-main {
  background-color: transparent;
}  
  
html::before {
  content: "";
  display: block;
  position: fixed;
  left: 0;
  top: 0;
  width: 100%;
  height: 100%;
  z-index: -10;
  background: url(https://us-east-1.linodeobjects.com/una/bx_posts_photos_resized/g/gh/ghm/ghmnxxsz2c7d6tkt3rtfchrreye6finw.png) no-repeat center center;
  -webkit-background-size: cover;
  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: cover;
}

#tipsandtricks