Well, when I got Charlee (rescue) I was told she was pee pad trained (they thought) however; in her foster home, I noticed both she and another puppy were "carpet" trained
As you know, this is always a difficult situation, no matter how hard you clean. I didn't have success with the pee pads, but I have with the Potty Park (with fake grass). At first I kept it in the apartment, but no matter what they say, I can smell it...so it is out on the balcony...she asks to go out, and out she goes. In the summer, I have a cat door in my screen and she has open access. Then she is 100% reliable. In the winter, like now, she is 95%, because if she needs to go through the night, she is just as happy with the carpet by the door...luckily enough though it only seems to be poos...so it is a little easier to keep clean.
Good luck finding what works!