I just pulled out my Sim card while the phone was on, then pushed it back in & let me tell you, my 4G signal is now strong, blue, and steady AND my gps lag is gone! You're probably wondering how I managed to do this, given the crowded / awkward configuration of the sd, sim, & battery. Well here's how 1st go into settings-storage-unmount sd card. 2nd pull off the back & slide out the sd card, then, without removing the battery slide the sim card out. Now it's not going to come all the way out. It's only going to come out about 1/32nd of an inch but it's just enough to disconnect it from the phone. So once that's done, & you see the red no signal icon in your notification bar, then put the sim back in, then put the sd back in, then reboot