I have now tried 2 different SiM cards from 2 different Cellular Operators, both SiM’s are confirmed to be working within another device. Although once the SiM is inserted and the Gateway is re-booted I am not able to connect to the Cellular Operator (Telstra nor Optus) I have checked the APN settings and these appear to be correct as they are working in the other device and can connect to the internet.
Please power off the gateway → reinsert the sim card → power on the gateway (the device does not support hot-swap for the sim card).
If this does not solve the issue, check the firmware version and make sure to flash the latest one.
If you already have the latest firmware make a factory reset.