No periods

I’ve had my IUD (Mirena) for 2 years and I haven’t had a period since getting it put in. Since I don’t have periods I have no clue when I ovulate or when I am fertile and I am trying to track it. What is the best way to do so? Cause with this app you have to enter when you start your period and I never do so I can’t get an accurate tracking. Needing some advice here ladies :)