I see. I Will look into this. Though i know on my verizon note 3, older modems caused slow speeds and bad coverage.There are PLENTY of odin-flashable stock restores here on XDA, just search for it. Flash any version below lollipop, unlock, and yes, the unlock will persist through flashing. It is permanent. You will need to stay on MJ4 modem for the sim card to be recognized though. Some ROMs require newer modems, thus making them incompatible with the unlock. You will need to play around with ROMs and see what works for you