I have a jailbroken AT&T iPhone 5S. I have had no issues until today. I noticed my phone had no signal on it at all, no carrier name or anything. So I ended up fixing it by resetting network settings. I wanted to go back to original OS, so I plugged into my Mac and whenever I plug my phone in, I get this pop up: iTunes cannot connect to the phone because an invalid response was received by the device. I cannot even get music over to the phone or anything. I tried putting it into DFU mode and I got an error as well. Tried a different cable and USB port, same error.