Hi Chris,
I have heard your problem happen more than once, and i have had to help fix your problem more than once.
For starters i doubt telstra will have anything to say to you other than its nbn's fault, hence me reply'n instead.
You can get your house changed back to an fixed wireless area by calling nbn and getting the wright person (not sue) adam's prob your best bet.
Next thing you need to access is if there is somewhere better for reception on your roof than where they tested the signal.
also did the tech offer you an extension pole to gain better signal?
Is there i hill near by your house on your property? i have seen plenty of photos of people getting there fixed wireless connections put in a garden shed or in extreme cases even shipping containers on top of hills or more elevated ares than your house and then just wifi the connection from the shed/shipping container to your house. you could cable it to your house, just cost more dollars.
Chances are you install guy/tech guy just put you in the to hard basket as they are only paid a certain amount per install regardless of how long it takes, so if yours was going to take 3hours then why not try the next house on the list that might only take an hour for the same money.
I recon you can get fixed wireless, you may just have to fight for it. in both cases i helped with it took 9months to get them back to the house and about 50hours on the phone.
helpfully you will have better luck, hope some of the information i provided helps to.
Rohan