Smart network switch only tells the phone to switch to LTE or 3G network if internet is not available on the connected wifi connection (e.g., when you are connected to a doorbell wifi network during setting up of a wifi doorbell). I would turn it off just to be sure; however, it should not have any impact to the wifi settings.
Things you can try that might fix your issue:
1. Restart your phone. Turn off your phone and remove the cover and battery for 10 seconds. Reinstall the battery and cover and press the power button to turn your phone on. See if you can toggle the Wifi setting on and off.
2. Backup all data and perform a factory reset to your phone. (Note: this step will remove all settings and accounts and wipe all the settings). You will have to reinstall all the apps and add all the accounts back into the phone. Usually this step will work unless there are apps that are turning those settings off.