I will have to agree with what b_p_smith posted because my experience has been similar. FWIW, I posted this same information yesterday, but a site mod deleted my post, so I will try again.
In my situation, I get zero bars for cell phone service inside my apartment, but outdoors at ground level I can get one or two bar service. I found that I could get three bar service from a neighbor's second floor balcony, so I installed my outdoor antenna on a mast that raised the external antenna to that height (a pic of my antenna installation which is mounted on the top of a kennel enclosure on my patio is shown below).
That arrangement has worked well for me. However, in your case (even if you can find a suitable place with a good signal for an external antenna) since you also mention that your current Sprint femtocell box does not cover your entire house, you might need to either install multiple repeaters or find a repeater with a higher signal output. There are high power repeaters available.