Outside Dog To Be Trained For Indoors?

Hi all, i just got a new cockapoo/chihuahua mix and was previously kept outdoors most of the time, i would like her to be an indoor dog because the fences we have are rod iron and she can get through, she is 1 1/2 years old. we also have a shih tzu mix about 2 years, female as well, but is a total indoor dog, the cockapoo goes potty outside, but i leash her so she won’t run away through the fence. she seems to always hang out by the glass sliding door leading to the backyard and does not seem to be interested in anything else, she seems very good natured and sweet though, any advice?
thanks.