I experienced the problem of no data connection after flashing a new rom on my Optimus G. The phone needs to be reprogrammed, updating your PRL and Profile will NOT help. You must dial ##72786# this will start the handsfree activation and your phone will reboot a few times. It may take a few seconds in between steps of the activation and it will seem like nothing is happening. Just be patient and wait for everything to complete. When it is finished your phone's data connection will be working properly.