Hi, I also posted in the MKR1400 forum that we need a sub-folder for the 1500.
I just got my 1500 last week -- using Hologram SIM on AT&T network.
Try these before your setup() -- this should get things going for you, hopefully.
#include <MKRNB.h>
NB nb(1); // 1 = debug mode - shows AT commands.
NBClient client;
GPRS gprs;