How do I get my iPhone to stop going into dark mode.

Disable Night Shift --> Night Shift on your iPhone, iPad, and iPod touch - Apple ...

Night Shift is not the same as Dark mode.

To turn off Dark Mode......

Tap Settings.

Tap Display & Brightness.

Tap Light to switch off Dark Mode.

If you have Automatic set to On......... turn that off also.

Settings>Display&Brightness>Light

Disable also option "Automatic" and "Night Shift"

When done, restart your iPhone.

Regards

Giulio