From your question you are probably using Lightroom Classic instead of the cloud version of Lightroom that this forum is for. No matter. The problem that you are running into is that when you export to dng, what you get is a raw file with instructions on how to render the raw data. Only Adobe software can recognize the edits. When you import the dng to the photoroll or the photos app on an iPad the iPad will use its own built-in raw rendering engine to render the raw data and it will ignore Lightroom's rendering instructions. If you however load the dngs in the Lightroom app on your mobile device, you will see it will render identical to the rendering in Lightroom Cassic on your dekstop.
So basically you have to export to a standard file format that is rendered already such as jpeg or tiff for the iPad to show you the image as you wanted it.