Hard to know when fertile days are around the corner??

Desiree • I'm a stay at home mom of four, and currently trying for a brother my eldest son who is 9.im a married women of three years, and I have three beautiful daughters.
Hey it's been a minute but I had my af a few days ago and it was unusually light and nothing but dark brown old blood just about the entire time and June's period was a no show due to vaginal infection and may was a week length, any ideas of when I should try getting a head start in baby making? My husband's job keeps him away from home alot and this app says around the first of August and online research says between 24-30 of this month and my body is strange so going by cm isn't all that reliable for me, Im thinking about getting opk but I'm unsure of when to try cause of his absence.. He's supposed to be home soon I just don't know when to try, any positive thoughts and vibes would be most welcome..