I noticed on the Telstra Website that Telstra is selling Fetch Mini Boxes for $96. I think it might be the new Fetch Mini 4K. Fetch Support confirmed that you can cancel the Fetch service with Telstra and link it to a retail account.
"Hi,
Thanks for reaching out.
If you have a Fetch box from Telstra and cancel your Telstra Fetch service, you can move your box to an account directly with Fetch (Retail Account).
All customer support will be managed by Fetch if you choose to do so.
The box warranty applies from the original date of purchase and will remain with Telstra. You can call Telstra on 13 22 00 to confirm your warranty period and cancel your Telstra Fetch service.
Unfortunately, you will not keep access to any channel pack or streaming service subscriptions which are part of your Telstra service.
Your movies and shows purchased under your cancelled Telstra account can not be transferred to Retail. Telstra Plus points are not accessible via Fetch retail accounts.
Let us know if you need any further clarification."
Edit: Telstra confirmed that the fetch mini is the Gen 5 (HY4424) aka the New Fetch Mini 4K.
https://files.ozbargain.com.au/upload/440408/114707/telstra_…
Also a person on Whrlpool had success in linking the Telstra fetch box to their retail account. They had to contact Telstra for them to deactivate the box and then they had to do a soft reset.
soft reset instructions:
Unplug power cable for 10 seconds, then plug back in
When you see the "Preparing System Please Wait…" message, start pressing the coloured buttons on the remote (Red > Green > Yellow > Blue) one at a time, repeatedly, and at a regular pace
When the "Checking Internet Connection" message appears, keep pressing the buttons, until the lights on the front of the Fetch box turn off, and the “Preparing System Please Wait…” message appears. This confirms that the reset has been completed, and you can stop pressing the coloured buttons
Edit 2: In case anyone was wondering, I bought a Fetch Mini from Telstra for my mum. After a quick call to Telstra to get them to deactivate the box, I was able to activate it with my retail account. So the Telstra fetch boxes can be unlocked. We are able to multiroom with our retail mighty and everything is working like a retail box.
I may be misunderstanding the wording, but the fetch websites says you can't mix n match boxes between an ISP (Telstra) and a retail Box (JB Hi Fi etc):
No, Multiroom requires that all boxes come from the same sources, and are on the same account and network. It is not possible to mix boxes from ISPs or retailers together.