Double check that you're running the latest version (settings>general>software update). iOS 11.4 is in beta, you might want to try
$1 or just wait for the final release, then upgrade to it; all works fine for me on the latest beta release.
Failing that, you can try resetting all settings (settings>general>reset) which clears preferences but not data.
If all else fails, do a restore in iTunes, then set up your device as new.