Puppy mills dogs are very difficult to housebreak. Remember, the poor thing has been forced to urinate and defecate in her cage all her life so the idea of holding it and/or going to a particular place is completely foreign to her.
You need to keep her confined unless you are with her. I'd suggest leashing her to you so can pick up on her signals and then take her to the place you have chosen (pads or outside) when she starts sniffing for a place to go. Then, when she goes, praise her and do the happy dance like you had just won the lottery!
Puppy mills dogs can be housebroken, but it will take a lot of work and even more patience on your part.