The Verizon phone uses CDMA so there is nowhere to put a GSM SIM. But a bit of hacking allowed him to add a SIM card slot to it. Even though he’s not the one who originally found this hack (XDA ...

This is enough to have your LTE network show up in the list of available networks on your phone. Of course, you won’t be able to connect to it yet — that’s where the SIM cards come into play.