Wifi

Settings>general>reset all network settings. Hold the sleep/wake and home button until the apple logo appears. You could try doing a restore from back up and then a restore to factory settings if need be. However, if this has been since the phone was new I would make a Genius Bar appointment at your local Apple Store.

Yeah, that would be quite the trip. MMaybe there is a authorized service provider near you: https://locate.apple.com/country

no resetting the network settings will only delete the network settings so you can reapply them

you may also check if your wifi router has a newer firmware available from the manufactor