HTTPS Error with GL7600

Hello,
I am trying to establish a HTTPS connection to ThingSpeak of MathWorks.
These are the commands I am using:

AT+KCNXCFG=1,“GPRS”,“TM”
OK
AT+KHTTPSCFG=1,“api.thingspeak.com”,443
+KHTTPSCFG: 1
OK
AT+KHTTPSHEADER?
OK
AT+KHTTPSGET=1,“/update?api_key=channelID&field1=1”
NO CARRIER
+KHTTPS_ERROR: 1,12

The error means “Validate server’s certificate error”.
The server’s root certificate is installed in the modem.
What could be the reason?

The modem connects flawlessly with HTTP via port 80.
Any help would be appreciated.

does your server need two-way authentication or one-way authentication?
you need to specify the sec_level in +KHTTPSCFG