Reading The Aetherium Wars may not add the miscellaneous objective 'Investigate Arkngthamz' to your quest log. I found these possible solutions on the UESP and TES wikis, however, I'm unable to confirm whether or not they work.
If you do this you will lose all items added by the expansion in your save and the DG DLC quest progress will reset, unfortunately. I mean, all Dawnguards' Armor sets and ancient weapons, all vampire armor sets and clothes. It's pretty annoying to get everything from this DLC.
