Audio hissing on 'S' sounds of speaking and in high points !!

Hi everyone! New and very basic editor here!

I'm having some audio issues and could really do with a simple quick fix (couldn't we all ahaha)

In simple terms, I'm editing a showreel together and one of my clips is filmed on an iPhone in a pretty empty room and has terrible audio. I obviously want it to sound as good as possible but it's mainly just hissing (and making fff sounds) when speaking 's' sounds or at louder moments in the clip. I tried using the denoiser and in my opinion that just made it worse, I've tried using the dialogue automatic settings in many different ways and I've even tried just making the 'ss' manually quieter- I am getting to my wits end with it to be honest!!

I expect I've done everything in the book wrong but it just sounds really bad especially next to clips with better audio- any basic suggestions of ways to improve it please!!!

Thank you everyone and hope you're having a lovely day :))