It appears that in order to use the the Hologram SIM with LTE Cat M1, you need to explicitly select the carrier.
You need to send
AT+UMNOPROF=xx command where xx is the profile number below.
Mobile Network Operator (MNO) configuration: * 0 (default value): SW default * 1: SIM ICCID select * 2: ATT * 3: Verizon * 4: Telstra * 5: TMO * 6: CT
You should send these three commands from the Cellular.on() state (but not connected):
int mnoProf = 3; Cellular.command("AT+COPS=2\r\n"); Cellular.command("AT+UMNOPROF=%d\r\n", mnoProf); Cellular.command("AT+CFUN=15\r\n");
The setting is saved in the NVRAM in the modem so you only need to do it once.
I was unable to use AT&T (2) with the Hologram SIM, though AT&T works great with the built-in Particle SIM.
Verizon (3), however did work for me (in the United States) with the Hologram SIM and a Boron LTE.