just back from the Telstra store. My FP5 was working fine until the 28th October, since then I’m getting the SIM1 not allowed error. I tried the support chat, first time ended up, they’ll get back to me. Second time, they told me I have to go to a store to get my phone unblocked. All bxxxxsxxx, they never got back to me and the shop couldn’t help me with that matter. They don’t even have a contact to their technical team.
Anyways, they checked an Optus SIM in my phone, which was working. At the end they suggested to write a complaint. They told me the matter will be read and hopefully forwarded to the right department. If somebody else wants to write a complaint, that might get some attention to the matter 
I do have an old 4G spare phone that I’m using as hotspot atm. I bought 6 months credit, so will use it for now. If nothing happens until Febuary, I’ll transfer my number to Optus as well.