Hi ladies

I am very confused by my apps my cycles vary from month tot month so Its difficult to exactly know when my fertile window starts and all the apps I have says different things and doesn't seem right to me so when should I actually start having sex to not miss the fertile days should I just start having sex when my period ends straight through? Last month my cycle was 27 days long this month it is 33 days long so I don't know when I am at fertile stage and apps all says different things so I am confused.