I realise at this point, this is not about Telstra, but it's related, so I'll post it here. I went away for 2 weeks, after calling Sierra again and being told that despite their belief that the issue is physical damage, they would send me a bag to send the modem back. I got back from my holiday and called them, the woman on the call said 'there's a note here that it's physical damage'. I reiterated all that I'd gone through and that I'd been told they would send a bag. She said she'd call me back after she spoke to a manager. It's been several days, they have not called me back.
At the core, the issue here is that Telstra sold something, and refuses to help me when it breaks and I need to get it fixed, they leave me to deal with the manufacturer directly. I can't recall the last time I bought something from a reseller and they did not offer to help me when I needed warranty service.