Tried both but i haven't that setting. I remember I used that feature in LineageOS 16.1, interesting. After your messege i was searching a solution and i've stumbled upon this page (under the post). A user says he has a screen resolution setting on klte. I haven't that either. Do you have that @Garry58? If you have that too, it's seems like i'll need a clean upgrade.
That's misleading. Display size is not a screen resolution. It just allows you to change text and other GUI elements larger or smaller. I personally use it at "smaller" setting, as it still readable and fits more information on the screen. It doesn't alter screen resolution, just changes the size of certain UI elements.
You can always change DPI through Developer options, but that's not screen resolution either.
I attached screenshots of both features. If you're missing those for some reason, I'd try making a backup, doing a clean install and maybe you could restore Data partition only. I'm no expert, but that makes sense to me. System partition would remain intact, while your personal data and apps will be restored (that doesn't include internal memory or external sd card). Worst case, you can restore the backup.
p.s. I don't find Ambient screen that useful on S5 tbh, since it has notification light, which has similar purpose (the color of which picked automatically based on app icons and can even be set by app manually. I didn't expect such customization on modern OS, as they're not typically installed on the newer devices).