What browser are you using on the Android tablets?
The special nip.io address you are using works around a weird quirk with Chrome allowing ‘Mixed Content’ when mixed content is explicitly allowed for a site, but the address is still fundamentally HTTP (not SSL) and each browser would need to explicitly be configured to allow the mixed content.
So if you try to use it on an Android Chrome version or Fully Kiosk Browser, you would still need to explicitly allow the mixed content (or otherwise properly support SSL).
The Google Chrome - Mixed Content - Media Not Showing link you mention specifically calls out these different browser types and approaches for each.