When trying to load a preset to Lightroom vie DNG, the DNG file is downloading to my phone with the file extension .dng.dng or .dng.tiff. I can see it as this when I open the image and hit info! Any information helps!
That's strange as the preset is downloaded with DNG.DNG or DNG.TIFF extension. Is it happening with all the presets? Have you checked the preset extension on Lightroom Classic or Lightroom Web( https://lightroom.adobe.com/libraries )?
Also, which version of Lightroom are you using on your desktop and using which OS?
Because you cannot install presets in Lightroom Mobile, preset authors are using this method to make it possible after all. What you need to do is import the DNG into Lightroom Mobile like any other image. It will appear with settings applied to the image. Save those settings as a new preset and you're done.