Thank you for reaching out!
It seems like at least for now, Telstra is using a 5G NSA network, which works by first connecting the device to an LTE cell, and then transferring to 5G. This means, that setting the device to 5G-only will not work, and it needs to be left either on Auto, or 4G + 5G.
Make sure there is a good connection and a 5G cell not too far.
If these steps don't help, I'd like you to navigate to System → Administration → Troubleshoot and generate a Troubleshoot file. It can be attached to your original post and will only be visible to Teltonika moderators. This will give me some more insights into the issue you're facing.