Apple watch SE3 for child unable to connect to mobile plan

I have spent 4 hours on the phone to a rather useless Apple Support, reset the watch 3 times and still stuck with the same issue that the watch does not let me connect to a separate mobile plan for my child. Every time I try to setup it tells me that my provider does not support it. But I do not want to use my provider. I have setup a plan with Better Roaming as per the Apple website and included the EID in the plan setup. I think the issue is a bug in the Apple software as it tries to connect to my mobile plan even though it is setup as a family member's watch! doing my head in!!!!