I'm in the same boat. The description of the alarm said it was GSM and GPRS compatible. I know GSM was on 2G but thought GPRS worked on 3G but my SIM cards ( one on Telstra and one on Optus) are both recognised by the alarm but won't connect to the network. I'm waiting for confirmation from the seller on the alarm below that it is guaranteed to work on Australian networks. I'll post here when I hear back. Their supplier is closed for Chinese New Year.

https://www.ebay.com.au/itm/2-4-Inch-TFT-4G-GSM-WIFI-Burglar-Alarm-System-Support-for-Alexa-Google-H...

I have one that  looks like this and it does everything I want except provide mobile broadband backup and SMS messages. Access via WiFi and the internet is fine.