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."
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.