Don't rely on an app predictions, they can be way out as they're based on averages. You need to learn to track. Your ovulation yourself using your temperature and discharge. I've done this for 10 years, but now use discharge only as I'm single and only tracking to know when my period will arrive.
You can ovulate anywhere from day 7 to day 65+ depending on when your body is ready. So the early part of your cycle can take some getting used to, depending on how regular you are. I strongly suggest you use condoms for the beginning part until you know your average ovulation date and are very aware that you can sometimes ovulate early. For example, I hadn't ovulated before day 14 in years and then all of a sudden i had a cycle where I ovulated at day 12. Then the next one was day 21. Due to stress.
The second part of your cycle is more fixed and can vary between 14 and 18 days usually. This is when you'll be able to have the most unprotected sex. Once you have tracked a few cycles and know your regular fertile period. Start but having unprotected sex the day before your period is due, so long as you have confirmed ovulation and work back from there each month to build confidence.
Once you're confident in tracking, you need to wait at least 2 days after you've ovulated before you can have unprotected sex. The egg can only live for 12-24 hours. So this prevents a mistaken ovulation day and having sex too soon.
In the early part of your cycle sperm can lay in wait for up to 5 days. Usually 2 though.
After I'd tracked a while my partner and I used no protection until day 10, then used withdrawal until v day 13, condoms until day 16, then no protection until my period. When we were happy with another child potentially coming along we just used withdrawal from day 12 to 16. I didn't get pregnant. But I'm not suggesting you use withdrawal as a method at all, I was also a good ten years older than you when we split up. You're in your peak fertile years. I'm just trying to give you my experience.
A great book that describes it all is taking charge of your fertility. And there was a Facebook group that I joined that was so helpful to help me when I got started. I think it was called the fertility awareness method.
I have a copy of the on vinted at the mo if you want it
https://www.vinted.co.uk/items/7994776167-taking-charge-of-your-fertility-toni-weschler
If you need any more help just shout, but you'll need to tag me as I don't seem to get notifications when someone quotes me.