To the bowl of a stand mixer, add the ingredients for the cinnamon roll dough: the water, milk, instant yeast, sugar, salt, dry potato flakes, eggs, shortening, butter, and half the flour. Mix with the dough hook attachment, on low speed until combined and smooth, about 1 minute.
While on low speed, gradually add the remaining flour, about 1/2 cup at a time. You may or may not need the entire amount of flour. You can stop the mixer and check the dough by lightly tapping on it with your fingertip - it should feel soft, slightly tacky, but not wet. *When in doubt, it’s always best to go lighter on the flour. You can always add more flour later, but you can’t take flour away. Too much flour in your dough will give you dry, dense rolls.
Knead the dough on low speed for 10-15 minutes, or until it's smooth and cohesive. It should be mostly cleaning the sides of the bowl. If you take a piece of dough, you should be able to stretch it a few inches apart without it tearing.