When out and about I use the Personal Hotspot feature of my iPhone 4 to let my MacBook Pro get online via the cellphone network. This creates a Wi-Fi link between the two devices, and is also known as tethering.
In recent months it hasn’t been working at all. I could connect to the iPhone but I would get a report that no Internet was available. Sometimes a USB cable connection worked, sometimes not. It was very frustrating and not what I expect of Apple products, although I was prepared to pin the blame on my cellphone provider.
I stumbled upon the solution in an unrelated Apple help forum posting — resetting the network settings, which can be done by opening Settings, then tapping General -> Reset -> Reset Network Settings.
Note: THIS WILL ALSO WIPE YOUR WI-FI PASSWORDS!
The iPhone will reboot, and presumably when it comes back online it relearns its APN settings from the SIM card or network (assuming you’re using a GSM iPhone, of course).
I suspect doing this each time iOS updates itself is a good idea, if we can remember.
Anyway, it works. I’m posting this now through an iPhone tethered connection.