Count Down Timer

Countdown timer- on my XR, what am I not understanding?
In my particular stuation, I’m in a gym, a loud environment, setting the timer for a stretching position, and with my ears sometimes less than a foot away from the phone, The screen goes dim, then dark, and I can’t here it at all. I don’t Care if the screen dims, but my timer should still be functional and as loud as I set it. There has to be a way to address this.

The sound is very soft, mulch lower than my Ringer volume. I’ve checked in Settings> Sounds and Haptics, the Ringer and Alert slider is all the way to the right, full volume.
Is there another setting that must be adjusted? I’ve looked at the Timer App and see nothing there. Maybe I overlooked it.
Thanks!

Thanks for your reply, but this confuses me.
I’ve used this feature before (previous iPhone same iOS) along with the alarm clock and in the past it made no different if the phone screen went dim/dark or not. You have a Timer going, you should not be expected to stay active on the phone to get whatever volume you set, should you? Imagine setting the timer, and then setting the phone down to watch TV and waiting for it to go off.

Of note, I’ve not yet tried the alarm clock on this phone.