I found the root cause of the issue.
sms_recv works correctly when using a different SIM card , but does not work with Vodafone prepaid (Oskarta) SIM.
So the problem is not MQTT or configuration, but SIM compatibility.
It seems that the modem does not trigger SMS notification in this case.
It looks like firmware only supports some SMS notification mode.
(I updated the firmware to the latest version as you wrote)
sms_recv works correctly when using a different SIM card , but does not work with Vodafone prepaid (Oskarta) SIM.
So the problem is not MQTT or configuration, but SIM compatibility.
It seems that the modem does not trigger SMS notification in this case.
It looks like firmware only supports some SMS notification mode.
(I updated the firmware to the latest version as you wrote)

