Cannot Re-enable developer settings after ROM re-install
Some ROMs move settings around. For example, on Ressurection Remix a lot of options are moved to new entry "Customisation" in Settings. Check your Settings app.

You can also use ADB or Terminal Emulator and run following command (taken from there):

am start -n com.android.settings/.DevelopmentSettings


You can also check your ROM's forum thread to see if anyone else also had problems with enabling Developer Settings.