Trying to use Import function and convert tif files from the LRT via LR to DNG. I checked the corresponding box, but always end up with.tif files?

You need to use Lightroom to convert tiff files to DNG. You cannot do this with the LRTimelapse importer.
There is an expert tips tutirial where I show how to convert jpgs to Dng. Do the same for tiff. See: https://LRTimelapse.com/tutorial/expert

Could you tell which of the advanced tutorials covers this conversion for DNG files?

It's the expert Tips tutorial #4.