Problem connecting child's se 23 apple watch to truphone data plan

I purchased a new apple watch se 2023 cellular for my son. We have set up the family sharing feature on my iphone 11 ( a work phone), purchased the truphone plan and then tried to go through the set up process which the phone prompted us to do, the phone displayed:

The telstra account on this iphone cannot be used to add a mobile data plan to Tasman's apple watch

The truphone website says:

If your iphone is locked or have an existing plan from a different mobile provider, you can still use truphone on your apple watch. The Truphone service is not dependent on the mobile provider used in your phone.

I have seen this massage has appeared for other people, Optus customer's too, anybody got a fix?