Mark got a new Google Nexus 5X and Sprint says they don't have that phone on their network. Leo says to just ignore that. All services have converted over to LTE and as such, they all have SIM cards. So if he gets a SIM card from them, it should work.
It's also odd because Sprint even shows how to program the 5X on their website! He may have to do some additional configuration of the data profile within the 5X. Mark should check out this tech note at support.sprint.com on how to do it.
Another possibility is that the radio frequency may not match.