By the way, I switched from using the AT&T SIM card I was initially trying to the Sierra Wireless smart SIM card we have to see if it makes any difference.
As far as I can tell, I am getting the exact same behavior though.
This is what wvdial shows (I ran this last night):
root@igt20:~# wvdial
--> WvDial: Internet dialer version 1.61
--> Cannot get information for serial port.
--> Initializing modem.
--> Sending: ATZ
ATZ
OK
--> Sending: ATQ0 V1 E1 S0=0 &C1 &D2 +FCLASS=0
ATQ0 V1 E1 S0=0 &C1 &D2 +FCLASS=0
OK
--> Sending: AT+CGDCONT=1,"IPV4V6","internet.sierrawireless.com"
AT+CGDCONT=1,"IPV4V6","internet.sierrawireless.com"
OK
--> Modem initialized.
--> Sending: ATDT*99***1#
--> Waiting for carrier.
ATDT*99***1#
CONNECT 150000000
--> Carrier detected. Starting PPP immediately.
--> Starting pppd at Tue Aug 21 18:58:00 2018
--> Pid of pppd: 1877
--> Using interface ppp0
--> pppd: `X▒
--> pppd: `X▒
--> pppd: `X▒
--> pppd: `X▒
--> pppd: `X▒
--> pppd: `X▒
--> local IP address 100.71.188.140
--> pppd: `X▒
--> remote IP address 10.64.64.64
--> pppd: `X▒
--> primary DNS address 8.8.8.8
--> pppd: `X▒
--> secondary DNS address 4.2.2.2
--> pppd: `X▒
I did find this forum post in which someone else mentioned that the MC7455 may not work for some carriers with the generic firmware (which I have): no cellular service for em7455 with window10!