No one can tell you when you're fertile - and definitely not an app of any sort. They just guess when you would be fertile going by your cycle length and the fact most women are meant to ovulate mid cycle. However, that is usually not the case.
You need to use OPKs to track when you're in your fertile time. I would start using them about day 10 of your cycle (i.e. 10 days since the first day of your period). Once you get a postive, then usually means you will ovulate within 24 hours. You should also be having sex every other day within this time. So sex every other day from about day 10- day 20 in your cycle will cover all those bases.
Usually you are fertile mid cycle which is 2 weeks since you had a period, download an ovulation app and put in your periods and it should say when you are fertile.x