Posting this for anyone else having this issue and is using Telstra (Australia) and searching this issue in the future.   We have hundreds of sites with backup connection via Telstra 4G Data SIM. Provided you are on the telstra.extranet APN. Things will work no issues because you get a proper public WAN IP. You need to ensure the Meraki is set to use the telstra.extranet APN. How to get telstra.extranet APN? You need to be on an enterprise or corp level account with Telstra and ask for telstra.extranet to be enabled on the service (magic code GPTEXB3 needs to be added to the service). Telstra by default will put you on a 10.x ip range as they don't give out a pulic WAN IP for  just any customer. If you are a Telstra retail customer. You will get CGNAT and you will have issues. ... View more