Go to Settings, and turn Airplane mode. After a couple of seconds, turn it off again.
Go to Settings, choose Privacy, tap on Location Services, and choose System Services. Turn off Wi-Fi networking services.
Go to Settings, click on General, Reset, and tap on Reset network settings.
Check the model of your modem, and use the browser to find the latest drivers.
Back up your data to iCloud first. Navigate to Settings, click on General, Reset, and choose to Erase all Content and Settings. This process may last only several minutes and in some cases an hour or more. It depends. Once it is all done, you will see the Apple logo. Set up your device as new.