The iOS Settings app is packed with a lot of settings that are just ready and waiting to be tweaked. And I'm a big believer in tweaking these settings to personalize and customize your iPhone for you.
There are some settings that you should just leave alone because messing with them can have big downsides. OK, your iPhone isn't going to explode, but messing with it could leave you tearing your hair out as to what's causing the problem.
Date & Time
Messing with your date or time is a fast-track way to have problems connecting to websites. And the annoying thing is that you won't get a message or notification telling you why you're having problems.
Settings > General > Date & Time
Leave this on for two reasons.
The first is that it does what it says on the tin -- it automatically controls brightness. This means it'll cleverly dim the display when you're in a dark room and brighten it up when it's a sunny day, and you might not otherwise be able to see the display.
Keeping this on is also a great battery saver, especially for people who otherwise would keep their display at full brightness.
Oh, and something to bear in mind about keeping your display on full brightness is that it can, over time, damage your display.
Settings > Accessibility > Display & Text Size > Auto-Brightness
Send as SMS
Remember SMS? Apple sort of blurred the line between iMessage and old-school SMS years ago, but it's important to bear in mind that people still use it and that if you fiddle with this setting, you'll no longer be able to send anyone an SMS.
So, keep this one turned on.
Settings > Messages > Send as SMS
Only change this if it's wrong. It doesn't change your actual phone number. In my experience, messing with this can cause iMessage activation problems.
Settings > Phone > My Number
Cellular Data Network
Yeah, just leave all this stuff alone unless you're instructed to change something by a tech support agent.
If you have messed things up, fortunately, there's a Reset Settings link at the bottom of the screen that will bring things back to a working state.
Settings > Cellular > Cellular Data Network