Hi sebastien.trocherie,
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HL7812 support Power Saving Mode, which is defined on 3GPP 27.007
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HL device provides connectivity with sim card inserted.
AT command can be used to make a connection to MQTT broker. You can refer this topic
TCP/SSL connection failure (HL7802) -
I think MCU is needed for sensor data. You can find example on this page. Note that AirVantage acts as a broker in this case:
Getting started with HL module on RaspberryPI or Linux-based system - av - Sierra Wireless -
I’m not familiar with PCB design. You could see the document here:
AirPrime - HL78xx - Customer Process Guidelines