What I meant was to try reflashing the firmware anyway. If its currently on the NC2 firmware, reflash that via Odin and then factory reset.
Especially since its refurbished, this will make sure they didn't make a mistake, or have something not work quite right when fixing the prior issues.
If after doing this, it still happens, try putting your own SIM in and see if it still happens. If it continues to disconnect after reflashing firmware and trying your SIM card, you may need to contact insurance and ask them to send one that actually works right as you have determined it is not the firmware or an account issue.