Apple watch Family Set-up not working with JB HiFi Telstra plan

When I try to set-up my child's new Apple watch SE cellular with family sharing I get a message that says "The Telstra account on the iphone cannot be used to add a mobile data plan to XXX's Apple watch." I am able to set up the watch and pair it to my phone but it can't connect to the cellular network. I have a JB HiFi Telstra plan.

I spent about an hour at JBHiFi and nobody there can help. Telstra doesn't want to know because the plan is through a third party. We have no Apple store in the city that I live in. Basically nobody can help me. Can somebody please tell me how I can find out if JBHiFi Telstra supports Family Sharing or any other work around.

Thanks.