How do I reduce photos on my phone to gain more device storage?

The virtually “no-pain” way is to use “Optimize” - offloading the full-size image file to iCloud Photos - if it hasn’t been already turned “on.”

How to check the storage on your iPhone and iPad – Apple Support (AU)

Set up and use iCloud Photos – Apple Support (AU)

The support article you came here from should provide you with the steps needed to try to resolve this issue. Have you tried them all yet? If so, and they didn't help, I suggest that you contact Apple Support directly. To do so, simply

select the "Support" link in the lower left-hand side of this forum's web page.