Video backgrounds are not available on mobile devices by default: most of users are not happy you start a video streaming that consume the monthly traffic or device battery for a visual background effect: this is note as a best practice in web development. Note also that not all mobile devices support autoplay videos background, although it's recently supported by the major mobile devices (recent version of iOS and Android).
With Uncode you can both set an autoplay self-hosted mobile video background or set a Media Poster image fallback for mobile video background:
Media Poster image for mobile video background
To enable the image fallback for video backgrounds please apply a Media Poster image. This method is necessary if you use a YouTube or Vimeo video (with these services is not possible to autoplay the video background on mobile):
Autoplay self-hosted mobile video background
To enable an autoplay self-hosted video background please activate the dedicated option: