How long after your period does your actual fertile days come?

I'm always trying to have sex on the days the <a href="https://play.google.com/store/apps/details?id=com.glow.android">glow app</a> tells me but no matter what I do whether it's everyday or every other day I still can't get pregnant 😢 I'm starting to believe my fur babies will be the only babies in my life 😭